That's partially a quirk of fast gentrification making housing super expensive, combined with lots of housing stock that fits well (row houses), and the fact that not many young people own cars, so parking isn't a problem.
Given the choice I'm sure virtually all of them would prefer a 1br apartment, but that starts at $1800+/mo in the desirable parts of the city.
Previously I lived in St. Louis, which is a much more spread out, cheap suburban city, and most of my friends had apartments.
If I had to guess I'd say the 'group house' model (whatever the local nickname) is predominant in New York, SF, DC, Seattle, and a few other expensive, dense cities, and rarer in the more suburban cities of the midwest/south. In NYC people often share small apartments since even a row house is out of reach.
That said, I think there is a distinct inflection point in people's lives here where they either partner up or just get tired of not having a space that is entirely their own. The 2nd seems to happen to 30-35 year olds a lot.
"Are these your dishes that were left in the sink?"
"I have someone coming over - can you go to the movies or something?"
I'm paying €650 for an en-suite room (no, not London!) and that includes all bills, sharing with 4 others. I'm one hour from London by train (~40 miles/65km)
Most one-bedroom flats in this area start at €975/mo, plus you need to pay €140/mo council tax and €100-150/mo bills (gas, water, electricity, Internet etc). Many couples rent for a long time before saving enough for a deposit to buy a flat/house.
It's terrible. House prices seem to rise by the amount of deposit you can save in a year...
My ideal situation would be to have my own bedroom/bathroom, high end furnishings, but a shared living space (living room/kitchen). But alas, most people have roommates entirely for economic reasons, so this seems to be tough to find.
Edit: When searching for a two bedroom apartment, I recall finding a good portion of the ads already had a second person living there who were looking to share.
Here in the Netherlands it happens but it doesn't seem to be the standard. It's extremely popular for students, and some of the leases remain after the students graduate and it effectively becomes a young-professional's home, and rooms get filled up when someone leaves, usually via FB connections. But actually applying to live with strangers say as a 29 year old is rare... either you have your own place, or one with a partner.
The reason I ask is because, theoretically and often in practice, shared living is more space-efficient. The living room is shared, the kitchen, the bathroom, the washer, all the little things, and then you attach 4 bedrooms to it, and you can get a footprint that's smaller than 4 studios with a similarly sized kitchen/bathroom etc. But it only works if, culturally, people can stomach living together with other people they're not family or intimate friends with. While undoubtedly many people want their own place, in a time (and city) of scarcity of space, I wonder if part of the solution should not simply come from shared living, rather than more space efficient solo-living. What's it like in your town?