Because they can - is the short answer.
That modern Western women are often attention junkies seems clear by their chosen behaviour - their clothes, hair, self-mutilations - the whole way they 'present themselves'... which shows that they strive to get attention; especially from men.
And this attention is sexual.
The 'attention' sought is mostly short-term, causal, superficial... the admiring glance, the second glance, the mild flirtation. Harmless, just fun, one might suppose? This is the affirmation of individuality in the mass context of the anonymous city. Attention is tangible evidence that you 'stand out' from the crowd, are an individual, are desirable to 'men'.
And when women seek more attention, they usually succeed - which is why the process is addictive.
Modern Western women mostly succeed in getting sexual attention because - so long as they are healthy - it is easy when they are young. This for straightforward biological and reproductive reasons: young and healthy-looking women are all desirable, all attention-grabbing. Some more than others, of course - but all young healthy women will attract attention.
So most women start with a baseline of un-earned attention simply by virtue of what they are when young: to men they are potential mates to be evaluated, to other women they are potential rivals. Thus the appraising glance, the second look, the atmosphere of flirtation.
And if young women want it they can get more, simply by sending out cues of 'sexual availability' which men are hard-wired to respond to - hence the use of revealing clothing, or signals such as tattoos and piercings, and the bold glance and confident self-presentation - which are interpreted by men as signals indicating a higher probability of low cost, no strings sex.
(Sluts always get attention; and a lot of modern women have noticed and taken this lesson to heart; and apply it to varying degrees.)
Of course, women may not be doing this consciously, they may deny that this is their intention, and they may be honest in this denial; but this is the reason for the extra attention.
And when more and more women are doing this, it creates an escalation of signalling, an 'arms race' among young women - to be 'the one' who grabs and holds most attention in a crowd. Anyone who opts out of the arms race becomes almost invisible in the crowds of modern life.
(In early human life, in small groups, every young healthy women was like a goddess due to sheer rarity value - she did not need to do anything at all: but simply be. Indeed it was her transcendent role to 'be'. As the poet Robert Graves put it: Man does, Woman is.)
So even young and beautiful women compete for attention nowadays, and because sexual advertisment is very effective at getting attention (attention from men, and also from other women who intuitively perceive a rival) then unless it is prevented by some internal restraint or external prohibition there is an innate tendency for women increasingly to sexualize their self-presentation - because if attention is the goal, then more sexualization works.
As women age into middle age - through their thirties - they progressively lose this unearned attention which is bestowed by evolution, and such women may start to complain they are 'being ignored'.
In reality, this 'being ignored' is merely a loss of biological privilege. (Welcome to the real world!) But in a secular and nihilistic world where community is broken down, and stable familes are increasingly rare; the loss of the daily thousand self-affirmations of casual attention is experienced as an existential crisis.
So middle aged women dye their hair to make it look younger - or maybe 'go blond', escalate the use of makeup, and start to dress more carefully to maximize their sexual assets.
And it works! They do indeed get more attention.
Perhaps for a while they get more attention because they accurately simulate the appearance of a younger women; but after a while they get more attention because they are giving-off signals of sexual availability - because the way men are 'wired' by evolution is that a middle aged (or elderly) women who is presented herself to enhance or simulate her sexuality, may be offering the chance of low cost, no strings sex.
Such signals are hard, perhaps impossible, to ignore - even when they are ignored or deplored. Hence the extra attention. For women; if you don't want to be ignored - and if that is your main objective, then the easiest and most effective actions is to make yourself look (more or less) like a slut - a woman of 'easy virtue' as they used to say.
(The woman need not deliver on what her appearance seems to promise - the advertisement of sexual availability may not be honest - but meanwhile she will get the attention she craves.)
In the end, women find themselves in an addictive cycle. In order to retain the unearned attention of their youth - or, as they may put it to themselves, in order not to 'be ignored'; from their thirties and continuing for longer and longer into the fifties, sixties, maybe even beyond - many modern women engage in escalating and un-discriminating levels of sexual signalling.
This is most obvious among single women and the divorced and those married women looking for affairs or new husband - but the addiction has infected even happily married women; who hate to 'be ignored' (as they perceive it) in the prevailing highly-sexualized social environment - and feel compelled to present themselves in a sexualized way.
This is experienced as 'empowering', revitalizing. It is socially admired. Instead of accepting their mature role as a 'mousy', ignored, middle aged housewife - faithfully devoted to family and community; middle aged women may suddenly dye their hair, or 'get a tattoo' - and experience a sudden increase in attention - especially from men - and all too often they misinterpret the real, underlying nature of and motivation for this attention. And other women will praise them (at least to their faces).
In the past, this arm race of sexual signalling was prevented by strong social norms - mostly enforced by the peer group of women - against even young women's behaviour. But especially middle aged women were sanctioned for presenting themselves in a sexualized way, or trying to simulate youth (the English expression for such behaviour was 'mutton dressed as lamb).
But typical modern women, and the mass media culture, do not enforce modesty and restraint. Instead our corrupt and morally-inverted society celebrates showing-off, attention-seeking: mere celebrity is celebrated.
To be unnoticed is to be despicable and despised
(I have heard middle aged women express real anger and resentment against those who do not join-in the attention arms race - who 'let themselves go' and accept the withdrawal of unearned privilege.)
This attention addiction is, I believe, the main basis of the ever-increasing sexualization of Western society over the past 50 years; and it was a natural and inevitable consequence of secularization and the removal of social norms that limited public sexual displays among women.
The process of societal sexualization has little or nothing to do with actual sex - the environment is highly sexualized, women are displaying, men are responding to these displays; marriages are delayed and broken, families are not had or abandoned under the pervasive pressure of 24/7 bombardment with sexual signals, of social life experienced as swimming through a sea of sexualization...
Meanwhile there is not much actual sex for most people most of the time - and never enough of it to build a life around (that being intrinsically impossible, a chimera) - and fewer and fewer children are born.
Western society spirals towards self-chosen extinction.
The sexuality of women is indeed a powerful social force - and any society which does not recognize that as a fact, and control this force, is doomed; as we are doomed.
At present each women is 'liberated' - i.e. allowed, encouraged, to manage this nuclear force of female sexuality entirely for herself. It is like giving a machine gun to a kid!
The problem is that the weapon of female sexuality will be used if it can be used without cost. Even with majority restraint, a minority can cause immense problems, and set-off an arms race.
Liberation is socially toxic: obviously so.
Female modesty just is a matter of general societal concern - and must in some way be externally regulated for the good of everyone.
There is no arguing with 'must' - but there are a wide range of possible means, some far preferable to others (some ways of controlling female sexuality are relatively benign, others are loathsome - the current Western secular attempts are both loathsome and ineffectual); and how best to regulate female sexuality is the proper topic.
Note added: It is my conviction, presented many times elsewhere on this blog; that problems such as the one described above can only constructively be solved on the other side of a mass Christian revival. Any attempt to solve such problems in a secular context, starting from where we are now, will almost certainly do more harm than good overall - because (here, now) our motivations are corrupted, and this corruption will out in attempts to address structural social problems.
The thing to bear in mind is that when there is a problem (such as modern sexuality in general) which is obvious, and which contravenes basic common sense and traditional wisdom - there must be very powerful forces maintaining that problem.
This means that attempts at reform must overcome strong resistance, which means that reform must be powerfully motivated - ie. reform must tap-into strong, widespread and fundamental human emotions.
In the modern and secular context, such strong, widespread and powerful emotions are almost entirely negative and selfish - for example: pride, envy, resentment, hatred, Schadenfreude. This seems obvious among the secular right wingers who advocate 'reform' in the sexual arena - either their motivations are too weak for them to succeed, or else their motivations are bad.
Only religion is capable of being both a sufficiently powerful and also good motivation as the basis for the tough work of reform. And of religions, only some few.
So, as in most things here and now, the first choice is to choose your religion. (And the choice of adequate religions, known to be sufficiently powerful, is in fact very limited indeed - indeed for the West the choice is practically limited to two broad and obvious categories.)
And if you cannot choose a religion, then (no matter how severe the problem - and indeed the more severe, obvious and simple-to-solve is the problem) it is probably best to do nothing: because no matter how bad things are - they can always get worse.