A major problem in American cities is what's called "contraception zoning." It's a de facto and sometimes de jure ban on large (as in 4 or more bedroom) apartments.
This constricts the larger apartment housing stock for families, and especially if you're living in the same room as someone else, probably makes your ability to cultivate a romantic relationship harder, too.
This adds up to - American cities ban units both large and small that allow a city to function properly; they simultaneously tend to dramatically underbuild housing altogether for the demand to live in these cities; and then we wonder why, given how prohibitively expensive it is to live in an American city, that they tend to have serious social problems and only the most tolerant/flexible to stressful situations—i.e. the most privileged—can afford and profit from living in them.
One thing I'm struck by living in Tokyo recently, aside seeing way more children than I ever do in SF (against all odds given the low birth rates), is that you can get a small studio for not much money - you'd be lucky to find one in SF within 30 minutes of the city for less than $2,500, but you can get one 15-20 mins from the hottest neighborhoods in Tokyo for like $500-$700. They'll be much smaller, but the lifestyle is also more space-efficient, so I find it to work for my needs as a 27 year old.
In other countries, large apartments are plentiful, and they are cheaper than houses the same size, because of economies of scale.