I was no particular fan of children or parenting before, but eventually I decided that raising children is such a significant part of the human experience, I'd be opting out on a huge chunk of what this life has to offer. Sure, it has major downsides and risks, but is also unmatched in its potential emotional and existential rewards. Going without may be perfectly rational for a given person, but for me, the opportunity cost tradeoff weighed in favor. That said, I became a father in my 40s after having had a pretty good run already, and that's definitely not something I would do any differently.

TL;DR: YOLO/FOMO.