The main reason is that they lack an underlying metaphysical system which would not merely notice things, but regard them as significant exemplifications of reality; then they lack the religious imperative to do anything about it (when doing anything would probably lead to short-term suffering with a high degree of probability).
So the blindness of modern Man is not difficult to understand - he has no reason not to be blind.
But what is God doing all this time? Modern Man avoids reality - yet reality is truth beauty, virtue in unity - unreality is evil... How come nearly everybody is living in a state of evil, and yet regarding themselves as Good?
Well, of course they don't really regard themselves as Good all the way down to the core of their true being - however their delusion does go a long way down; probably far enough that they would choose to reject Christ even if 100% convinced of his reality and truth - they want damnation - want to reject eternal life in Heaven, because ultimately they prefer it.
But again why? Or, how could it be expected that people be better in a society of such pervasive corruption? When all the leaders of all the institutions are corrupt? When the mass media, and the linked bureaucracy of government, corporations, law, police etc... are all corrupt. When the mass media and bureaucracy are united in propagating corruption, pushing surveillance and micro-control, punishing deviation...
Why has God allowed such circumstances to develop - and at the same time be 'invisible' to the great mass of the hedonic-utilitarian soul-denying populace?
We are being pushed back and back, being stripped of all valid authority, the churches are doing more harm than good, we have nowhere to turn...
Until we recognise that we have everything we need, available to us each and personally. And that is the point.
We need to be weaned of passivity, dependence, secondary-ness - we need to know all directly, for ourselves, by experience. And we are being forced-back into exactly that situation...
Eventually we will be confronted by stark reality, and the reality of our own capacity, our adequacy, the adequacy of Life for its necessity - and will choose then either to live by reality, or against it.
Note added: My assumption is that God ultimately intends for us to become like him in nature, which means an evolution (theosis) towards autonomous agency; towards knowing directly and for-ourselves, rather than indirectly from others via communications. This is more a question of shifting the balance between ways of knowing, than it is a matter of either/ or, all-or-nothing. But part of becoming spiritually grown-up is the transition from immersion-in and child-like passive obedience to external communications from God - towards active-knowing-of Reality based on that which is divine-in-us; and conscious choice in-favour-of working-with God. A move from unconsciously participating in God's creation; to consciously participating in the work of God on creation.
Further note: We really must hold-onto the fact that the reason why people are so blind is Not that they don't know; the answer is not that people be shown - the answer must go much deeper: the answer must go as deep as revising our primary and most fundamental assumptions. Only then will seeing lead to knowing.