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.
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.
Apartments are for dense housing, big apartments aren't dense, therefore look elsewhere for your large apartment, I guess. You could have 4 tiny bedrooms, I guess, but that's weird too.
It's nice to have an office/guest room, but if you're raising a family that needs three bedrooms in apartments, you're compromising.
The reasons vary, but I have lived in Boston/Cambridge area for 40 years and can’t think of literally any of my friends who raised a kid past the age of 3 in an apartment, despite many of them (including me) enjoying the apartment life while young and single. But, literally as soon as we could [barely] afford to, we bought a house and only then added to the family.