Or worse, a new owner might tear it down. Then the ghosts would be living outside.
In my mind, the reason they sit, sleep, walk, etc., is because they behave the way they are used to from when they were alive. So it's more of a mental thing. It's only when they try to physically touch something that they go thru it. So when they are sitting on furniture or walking across the floor, they are not actually touching those things, they are levitating and they just don't realize it because their astral bodies are mimicking whatever habits they are used to.