Earlier quoted context omitted.
There's this unfortunately prevailing attitude that you aren't successful in America if you aren't living in New York or San Francisco. It's absolutely bonkers. There are many other great places in this country besides those two cities.
These days, it's not enough to live in a world-class city, you need to have your own kitchen, living room, and bathroom that barely get used. The people I know who complain about high cost of living refuse to live with other people.
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.