You were left alone so abruptly that you decided to float out of the wagon. Of course, since you're a ghost, you just float through the wall or something. Why even bother with doors as a ghost. You mean, what purpose does it serve those who can bypass it. Those who can't even use them. That's why you were so flippant in the underground. What are they going to do, lock their doors? You can simply float through it.
You suppose that the door on houses of ghosts can be used by guests, much like a guest room in a larger house does. But you don't want any of that. You don't particularly like guests. You've only had 2 non-family guests, that being the human and Mew Mew's bird friend. Otherwise, it's a useless thing for you to have to have on your wall. It's a bit of an eyesore, if you had eyes. If you could have it how you wanted, you would have no doors, and you would have all the things you could ever want (ghost food) right inside your house. Of course, you'd probably go insane for a bit, but aren't all ghosts a little insane?