Perhaps because regardless of some policies targeting the economics/material issues of having kids that some countries have put in place (Russia, Hungary, S. Korea, etc.), those policies don't change the brutal cultural situations in those countries: 2 authoritarian regimes + extreme competitiveness of S. Korea.
It seems like the neoliberal era growth model has long since broken down, and our politics haven't adapted with deeper reforms to reflect the latest technologies, etc. that could produce more material abundance than today (see: China's rise).
As a left accelerationist, I can't help but think that a society with a 4 day week, no precarity, cheap access to good housing, healthcare, education, etc. would increase the social trust and the birth rate, as most people want kids and want them to grow up in a thriving society.