Premade houses

You have your solution. But the explanation is that houses and other buildings that sims can actually enter (as opposed to rabbithole shells) are not considered objects in the game that can be moved around and placed. What we are actually placing from the Edit Town bins are entire residential or community lots that happen to contain houses or other enterable buildings. I know that sounds like mere semantics, but sims cannot own a lot that is then replaced with another lot, they have to be evicted or somehow disengaged from owning the lot that is to be deleted before a new entire lot can be placed for purchase.