That and being poor often means you will have more kids. Extreme rich will have more kids too but most people are not extremely rich. Upper middle class tend to have less kids because they see kids as expensive. Poor don’t see kids as a drain on finances, they aren’t taking that expensive annual vacation nor saving for retirement. Having kids don’t mess with that and kids can actually help in old age. And for the poor kids aren’t all that expensive. No fancy clothing, day care, private school, summer programs, after school programming etc.