Earlier quoted context omitted.
In the US the nuclear family is usually two parents and two children. The parents share one bedroom and each child gets one. In Asia I would guess families make room for their older relatives which would mean an extra bedroom is needed. Immigrants to the US typically do that but I think it's uncommon among the rank and file Americans.
I think in the US a home office and a guest room is also usually needed, so 4-5 bedrooms makes total sense. In fact 4-5 bedroom single family houses in the US are extremely common. What I don't understand is why apartments always seem to stop at 2b or 3b.
Houses, on the other hand, have 4 external walls at the very least. Not to mention most 4+ bedroom houses are 2+ stories as well.
[+] It might be also required to have staircases at decent distance, as a second staircase going through the same stack of apartments is not of much use as it's likely to be blocked by fire at the same time as the other one.