I mean, in theory there could be acceptable ways to let a couple with babies have roommates. But the design problems alone seem too complicated to attempt, let alone the cultural problems.

Also, keep in mind that a young couple with kids might have only one income earner. And that couple is in the same apartment hunt with singles willing to share multi-tenant residences. Sometimes three or four of them!

Point being, "get roommates" doesn't scale over time or over the entire population.

No, just rent and share like a college student your whole life. That makes sense.