Hot take: this isnt because of capitalism.

We've had capitalism for a while now. Birth rates were higher during the robber baron era.

It isn't because raising kids is hard now. People had kids in Dickensien England when they worked 16 hour days, 6 days a week, in the get-kicked-in-the-nuts factory.

Its not because the future looks bleak. Native tribes in the far north have been reproducing for millenia, despite knowing for a fact that they will suffer perpetual night and horrifically cold temperatures with scarce food and not enough warmth for every winter. People also had kids during the cold war, when many had legitimate fears of nuclear annihilation.

Instead, it is because (1) people have a choice, and more are choosing no, and (2) because people are not indoctrinated with the belief that having children is their purpose in life anymore.

(1) is due to condoms, birth control, and abortions. Remove these things, as in the past when they were not available or were significantly less available, and birth rates will go up. Many would be unhappy about this.... but birth rates would go up.

(2) is due to the breakdown of organized religion, communities, and family structures. The church told people - women especially - that their job was to make as many babies as possible. But fewer people go to church these days, and having children or having a career or having children and a career is considered a choice that any woman is free to make without judgement. And with family and community setting an example, it is both visibly more possible to care for children and also more pressured. If you are a young woman in 1950, and you go to church every sunday where they tell you your purpose and responsibility is to have children, and all your friends are having children, and you saw your mom raise 12 kids in a shotgun shack, then you too will feel like you should start popping out kids regardless of your life circumstance.