It's just not cool to have kids. There are many more ways to have fun and status in society, so having kids is either coming as a social burden ("i am expected to by my spouse/relatives"), or a religious thing. Rationally, it's such a pain in the ass to have kids, while you can have some much more fun without them: travel the world, meet people, learn and explore! Clearly, having kids is net cost and suffering.

Yet, those who opt in do have a different opinion. We got two a decade ago, and then a couple years ago through of FOMO that when we are 45 we'd look back and regret missing the window of having another couple of kids. So we did. I'm 39, have four kids, had to get a bigger car, pay the airline tickets through the nose, spend a lot of time on kids' stuff, and love it. My family is the center of the universe and I'm the happiest and wisest dad alive. Everyone else is childish ;-P