Poor people have more kids. Look at developing countries especially. Low fertility is a problem of the affluent. That's part of why I say I think it's cultural, it's seen as such a negative (including as a financial burden) instead of as a positive thing to be aspired to, at least that's what's pushed on younger people.

That's no longer seem to be the case in the US, especially as out of wedlock births decreased and poor people are less likely to get married

https://ifstudies.org/blog/no-ring-no-baby

I don't buy this argument.

Historically, people in the countries that now have low fertility rates were much poorer and yet we're having far more children.

In fact, the countries with the highest fertility rates globally are very poor.

Historically, poor people's children started being "profitable" (as in doing more work than they take) much more quickly - they were raised communally (spreading the required work evenly), and they started working and helping earlier. This made having many children a legitimate strategy to deal with poverty. This has completely turned around - community is almost non-existent, and children will really only be "profitable" once they are adults.

The situations are simply too different for your argument to make sense.

Fair point, but I think you also overestimate how many people make decisions using actual cost-benefit analysis.