Why is it necessary to own a house before you can have children?

That was not the case for the families I knew growing up; homeownership was the exception, not the rule. My parents had eleven children, and never got around to buying a house.

Did they at least rent a house? If not, how spacious was their apartment, how accessible was outside space for the kids to play in? If so, what was the percentage of their income was the rent?

I do think a lot of young people who grew up in suburbs mistakenly believe they need a house before they can have a kid, however, for those who have wrapped their heads around having a baby in an apartment space and cost is STILL a limiting factor. A 2nd bedroom is a significant cost increase, and you're competing with a huge market of people (young families, WFH ppl with home office, roommate setups etc). Also, once you outgrow that 2nd bedroom with kid #2 you have to move. We don't build 3-5 family bedroom apartments or condos or town-homes near job centers.