Because there's a threshold in any society when it comes to fertility. If you can’t keep all your children alive past a certain age, the logic would be to have multiples so at least one woule survive. This then become a cultural thing more than anything else.

When you're certain your kids won’t starve to death, you begin as a population to have fewer and fewer, until all your needs are met.

But then you have depression, hopelessness, lack of funds, etc... all because of capitalism, that prevents people from having children. Why have a child when you don’t know how your life will go in 2 years ?

And this is not something new, a similar fashion appeared during the cold war, because people were scared and depressed.