[bobthepanda]

US flats generally have a lot more due to different social standards; the in-unit washer/dryer is not common globally, for example. Or apartments with double vanity bathrooms.

The families thing is also because all other things being equal, more bedrooms will lower the price per sq ft of a unit due to weaker demand; and right now studio and one-bedroom demand is so high that you could reasonably build a building full of them and generate profits.