If you make kids share a bedroom it drastically decreases the margin for tension between children because they don't have anywhere else. That can work, of course, but it might not and too fucking late for the kids if it doesn't. That can mean physical abuse, but it can mean things like one kid loves loud music and the other wants to read quietly alone, or their sleep schedules naturally don't align well - if you were an adult house share you'd say well, we're just incompatible, it's nobody's fault, I'll move out, but kids can't do that, they are stuck with the situation their parents created and it's all they know.

My mother - I found out years after I'd left home - was worried that I resented the fact I had a small bedroom while my younger sister got a larger one - but in reality I didn't care at all, she's an artist, she makes stuff which actually exists, of course she needs space; I write software, which conveniently takes up no space, whereas if I'd had to share with her that would be extremely problematic and wouldn't have gone well. I could be in my tiny room and that was enough.

Raising one child is fine, raise two or three means sharing rooms not having a guest room (e.g. relatives can't stay easily) or an at-home office space.

It's not that you can't, it's just that it's not the standard of living most western people expect.

It depends on how many kids you have, no? I did know other kids when I was growing up who shared a room with one or more siblings, but some parents want to be able to give each child their own bedroom.

So a three-bedroom apartment might not be enough if you have three kids.