My hypothesis is that human progress (social, technological, moral, economic) is by definition at odds with having children. The more advanced we get as species, the less attractive it becomes (for most) to have children (many, or at all). And there’s nothing strange about it. You could even say: The individual’s need to have children - many or at all - was a bug in the human operative system, born out of circumstances that are being eradicated with human advancement. And I’m meaning this completely dispassionately, not in any way labeling this as good or bad.

One could call it paradox of human progress. We are advancing to the point at which we eliminate the process that so far has sustained us as a species. The only ways out I see are that existing human lifespans are expanded significantly (with all the challenges this brings), or that childbirth is being entirely/partially offloaded from women to private/government-run incubating machines (sounds dystopian, I know).