It's just a curious question ^^;. Like they all have a basic 1 room home, and it'd be cool if they also upgraded their homes. (Maybe an achievement of some sort would make your neighbor upgrade their home). That seemed like a neat idea.
I'm guessing its just so that if they move in somewhere inconvenient, because they can move in pretty much anywhere in town, it doesn't take up more room and make things worse. Though it would be cool if their houses improved the longer they stayed in your town.
But it might also be an issue with number of furniture? Having bigger houses for everyone would more than double the number of items that need to be generated for villagers, I think. Plus, just imagine trading and swapping furniture even twice as much as before o_o
I like to imagine they do have bedrooms and bathrooms etc, but we just dont get to see them, i mean if you go to visit someones home, you're likely going to stay in the living room, and not go in their bedrooms etc..
Probably because I remember a while ago I remember my old villager Biskit asking me to buy something and saying something like "I have 100 bells on me and that's my life's savings." These villagers can't afford that stuff