It all comes back to women being treated as full people. Having a child is dangerous, expensive, and a major time commitment which mean that women who have other options are going to have fewer children later in life when they have the resources to support them. We also have much less demand for unskilled workers so even women who really want children are getting educated and establishing careers first rather than getting married at 18.

https://publichealth.jhu.edu/2026/is-the-us-birth-rate-decli...

That leaves really only two choices: pull a Ceaușescu and try to remove the choice, or improve all of the things which make people feel now is not the right time to have kids. Since the former choice is both immoral and self-defeating, that really flips the discussion to why the people who claim to want more children oppose universal healthcare, childcare, making housing more affordable, banning negative career impacts for mothers, addressing climate change, etc. There are many things which factor into an expensive multi-decade bet and you have to improve all of them to substantially shift the outcome.