A wider choice of mates reduces people's reproductive output.
economist.com
economist.com
Within a larger organism, there's no surprise individual cells don't wildly reproduce in the organism's warm, safe, nutrient-rich environment. That's against the conditions of formation/inclusion. When it happens, it's often a cancer that threatens the whole.
the sort of good values that make societies richer include caring about your children individually and spending much more money per child, instead of just trying to have a lot of kids and use them for cheap labor.
also, our culture considers children a pain in the ass. then people wonder why we aren't having as many as we used to? because more people are doing what they want instead of blindly following tradition.