I think it's the contrast between what people want for themselves and what brings status. Maintaining status while raising kids--and having status for the kid y

I think it's the contrast between what people want for themselves and what brings status. Maintaining status while raising kids--and having status for the kid you're raising, like sports teams or good schools--is expensive. Status is how society signals you should do something, so it's a strong incentive. The situation, at least in NYC where I live, seems reversed for the very rich, where more kids is a clear signal that you have a lot of money and yet spare time for your kids.