They think you’re missing out and might regret it later. Mostly, I think they are trying to be helpful.
A few people maybe just can’t understand it, like “doesn’t everyone want kids,” or they judge that you might have a dark soul, but I think that’s not very often.
I have to admit I find it disconcerting when people hate children, but to me that is completely separate from not wanting to have children or simply never having had children. Maybe some people assume that if someone didn’t have children they don’t like children?