Top income brackets aren't really having more than 2 children either, which is a requirement for growing population. Like most studies has shown that, in general, educated women, freedom of choice and etc. will negatively impact birthrates. It's the same thing everywhere. Sure, income, less social pressure and etc. affects it somehow, but there's just no real need in general to have 3 kids in this day and age. Asking a woman to give away at the bare minimum 6 years of their youth won't cut it nowadays. And honestly, I don't blame them, I think exactly the same way.