This doesn't fully explain why humans don't want to have a lot of kids. Have you ever been around kids? They're really a pain in the ass, especially if they're boys. (For instance, as a single guy, I'm
far more likely to be interested in dating a single mother with girls than one with boys; little boys are just too destructive and disobedient.)
Smart people aren't having so many kids now because they don't have to. If they don't have a religion telling them they need to, and they have a good career and a very comfortable lifestyle that affords, why would they want to go to all the trouble that raising a lot of kids requires?
In addition to that, kids are very costly in today's society, and with adults not living near their families so much, you don't get any help with your kids when you need it, so that makes it even more unattractive.
Having a lot of kids made sense back in the days when you needed the help on your farm, and your whole extended family (or village) lived together and could collectively share the burdens of child-rearing. Now, it just doesn't.
The answer to me is simple: we need to outsource child-rearing to the State, as shown in the novel "Brave New World" where children were made in factories and then raised by trained professionals.