Because below a certain agricultural productivity level having more kids is actually beneficial for your economic success. Once you get over a certain productivity level it makes sense to have fewer kids and invest in them moving up the economic ladder.
>Builders in Arkensas have more kids than tech workers in San Francisco?
Because the cost of every warm body in a household in SF is completely asinine so people have fewer kids than they would like to.
Poor people everywhere have more kids than richer people in the same places because various welfare programs make the net cost of each kid less. There's also a massive cost cliff once you get below the threshold at which parents have enough income to justify saving for their kids college.
That doesn't mean family sizes aren't smaller in high cost areas. This is a well known trend.
Are we talking locally rich or globally rich?
People in industrialized (rich) nations don't have tons of kids because that's not how you get ahead in that kind of economy. Subsistence farmers have tons of kids because that's how they get ahead economically.
Within richer nations richer people have fewer kids because the way social safety nets and social expectations of how one should raise one's kids make kids more expensive the higher up you go.
I'm skeptical of local explanations because the relationship is so universal.
https://www.gatesnotes.com/about-bill-gates/a-conversation-w...
I just don't buy that very poor people ration out sex under a precise calculation of how many kids they want. Doesn't fit my observations of how humans behave. Most people fuck because they're horny, or because their husband is horny and they need his support to survive, or frankly because they're so poor they have to sell themselves. HIV is a bigger risk than not having children in old age, most Africans know that, but that doesn't stop it either.
The life choices of women are an important factor. Having both a career and children is still difficult today and more women choose careers.
The reason peasants in Africa have more children is manifold: from religion and lack of knowledge preventing contraception to outright necessity, because in these regions child labour is a thing.
A for developed countries: again, multiple reasons with religion still being right up there.
Why do peasants in Africa have more children than Lawyers in New York? Builders in Arkensas have more kids than tech workers in San Francisco?
Poor people in general have more kids than rich people. It doesn't fit the data.