I hypothesize that it is careers. 1. You probably want to establish your career before having kids. That takes time, especially with all the training required

I hypothesize that it is careers. 1. You probably want to establish your career before having kids. That takes time, especially with all the training required and needed mobility. 2. Kids need to have a good career to have a nice life, so you need to invest more in your children, which means fewer of them.