>>38413774People had lots of babies way back when because it meant more people to work your farm/self owned business to support you when you got old. Now that you don't need a bunch of kids to sustain yourself and things like contraceptives are readily available, it comes down to couples deciding how many kids they want. And most people don't want the responsibility of caring for 4+ kids.
Add onto that the fact that education demands are getting higher so people are getting started on their careers later and therefore starting their families in earnest later, that means less time for people to have kids even if they wanted to. It's not all that abnormal for a woman to start thinking about kids in her 30's, which is rapidly approaching the end of her ability to crank out more than 1-2 kids.
Ultimately, as civilization evolves, mass producing kids is just less viable, and less advantageous.
