It's a combination of desperation, fear, and willful ignorance (one might even say self-induced stupidity). You've had forty years of propaganda at this point, and the old management who designed that propaganda for their followers are retiring or dying and being replaced by younger generations who were or are the followers of that propaganda. In essence these newer generations don't know the lies aren't real, and have been in so deep for most of their lives that they refuse to shatter their worldviews. And so when those worldviews keep being shut down or outright disproven they become more reactive, doing anything they can to keep their understanding of the world intact. They believe that their opposition are creating lies and using systems of information and governance to support those lies, and so to destroy their opposition they are willing to destroy the very systems they themselves work in and rely on to dispel the supposed lies.

They believe they are the only sane men and women in a world of shadows and madness because that's the only way they can continue to function. There is no reasoning with someone who's gone that far. The veneer of egalitarianism that politics in the U.S. has prevents anyone from actually doing anything to stop this increasingly volatile group lest the intervening mediators be seen by their peers both within the volatile group and without as aggressive savages who would subvert well established conventions. The potential mediators must let themselves be stabbed and reply with "That is quite impolite" or else they lose all trust from said peers.

> Not just chastity but also poverty.

Virtually every cultural subgroup in history had their birthrates plummet once they reached a certain amount of wealth. Evidence indicates that if anything, poverty is good for population growth.

But how does that work out in places with untreated endemic HIV?

>Not chastity but also poverty

I never see poor people collectively failing to have children.

I only ever see wealthy people complain they aren't wealthy enough to have children and once they have more money that's when they'll have children.

It doesn't actually take very much money at all to have a child if you have no standards.