I used to think it was only because of selfishness, but I think some people just believe there's enough good in the world that having children seems reasonable. There's definitely a lot of selfish parents out there, but I know that isn't all of them, it really depends on the parent.
In the past, I definitely hated all parents of biological children, but recently I've started to think that it's unfair to do so. The only thing any of us can really say is whether or not our own parents were selfish or not, but it's hard to really know that about others unless they tell us their reasons. In my opinion, having children seems like a bad idea regardless, even if it's just for the fact that unfortunate things happen to people all the time, and even the best, most selfless and loving parents can't protect their kids from it every time, but maybe some parents believe they can, I don't know.
Regardless of whether having children is moral or immoral, I'd think that the risk of losing a child to something horrible would be a good enough of a reason not to have one right there. Imagine being the parent of a child who is dying or has died from cancer and/or suffered immensely from it (or any other disease). I'd be curious to hear the perspective of someone who has gone through that, and whether or not that experience has caused them to regret their decision to become a parent.
HopefulSleep, good on you for not judging or hating people just for having kids. I used to be one of those anti-natalists who hated people with kids, but realized that I was just projecting my hatred for my own emotionally abusive father onto other parents, and assuming they were all like him. That's not the right way to go about it, and doing so accomplishes nothing (with the exception of raising my blood pressure to unsafe levels lmao).