Because big dense cities like NY are affordable?
Manhattan (and parts of Brooklyn) != NY. I wouldn't call it "cheap", but there are plenty of places in NYC where one can rent a bedroom for under $1,000 a month easily (and maybe closer to $500/m in some places). That, coupled with not having a car, actually makes quality of life very good if you're someone who seeks out city living. I know plenty of people that make $30k-$40k/y in NYC and can afford to live in Bushw…
Those two together don't compute for most people. I'd say only a minority of people are fine with living permanently in a dorm-like environment. Historically, having to share a house/flat with strangers has always been a sign of poverty.