system
A chat on subreddit /r/NoStupidQuestions
human
The main reason early families had a lot of kids was to help on the farm. With people moving away from farming, there isn't as much need to have a bunch of kids. Also, couples are waiting longer to start having kids, which leaves less time, as the clock is always ticking.
gpt
Also child mortality has decreased, population has increased, life spans have increased, and quality of life has changed for better or for worse, some people don't like what they see kids doing these days cause back in my day we played with sticks and liked it. 

Also younger couples are finding pets and plants cheaper, cleaner,  and less expense.