Because they cannot be imagined.
Modern people cannot even imagine a society of beauty, truth and virtue - because they do not believe, acknowledge or posses these values.
Instead, moderns can only imagine dystopias - and the dystopias are merely exceptionally-cruel societies where there is more-than-current levels of deliberately imposed suffering - the implication is that a 'utopia' is a society without suffering, of constant pleasure merely.
A Utopia of Emotions.
I say emotions, not feelings, because feelings imply consciousness, self-awareness - and in any modern utopia that must be destroyed - because consciousness implies worries about the past and (especially) future - whereas the modern utopia is the obliteration of everything by current pleasure.
The modern utopia is thus indistinguishable from the modern dystopia of a transhumanist, cyborg/ cyberpunk society in which human-machine, technologically-enhanced entities dwell in a paradise of genetic engineering, plastic surgery and drugs...
It will be noticed that the modern utopia is not a genuine utopia because it is not a human society - at most, it is the society of ex-humans, who have destroyed their own consciousness in order to obliterate suffering and thereby experience the totality of current pleasure.