for me personally the problem is that i don't really have local friends, let alone ones i'd want to buy a house with (or the money to pay the down payment on a house of any size, i suppose, but the multiple people would presumably help with that part)
the specification of separate flats is, i guess it could be summarised as "personal space needs", as well as kinda like, limiting friction. there's people who are great housemates to live with and i don't think i'm one of them, but if shared space is more limited i could do my share of the chores there.
and just like, i can imagine people who i could hang out with every day but who i wouldn't exactly want to live in the same two-bedroom with. letting more of the space be optionally shared rather than always shared just feels better to me.
the realistic option here is probably either having multiple flats but they're shared between two or three people (i know some arrangements like this but they're rather special), or only having individual rooms but there's still multiple bathrooms and kitchens and such and you try to arrange people to cause the least trouble