Two reasons:
1. It’s the current fad, and they want to signal their virtue.
2. They are obsessed with the abnormal.
A freeper recently posted an essay by Joseph Sobran,written in 1985 which lays out the modern liberal with uncanny prescience:
For the modern liberal, who is essentially a man of the Left, the immediate has apocalyptic urgency. He is an active member of the Cause-of-the-Month Club, forever prescribing drastic action to prevent the world from being blown up, overpopulated, poisoned, oppressed, or exploited. He thinks a government that maintains law and order — a big job at any time — is “doing nothing”; because to his mind a steady and quiet activity is nothing more than inactivity. Though he speaks the language of environmental preservation well enough, he never pauses to imagine the “environmental impact” of his own policies on a social ecology that is, after all, no less real because he disregards it.
In short, he is always sacrificing the normal (he is barely aware of it, or sneers at it as “bourgeois”) to the abnormal. Life, to him, is a series of crises, inseparable from politics. He is too concerned about our “rights” to bother about our health — rather as if a man dying of cirrhosis were to toast the repeal of Prohibition. If he ever has moments of well-being outside of politics, he has no vocabulary in which to talk about them.
But what you've posted by Sobran absolutely nails today.
Joseph Sobran was a true national treasure, run out of the conservative movement by people unfit to refill his ink-well.