It's because as Vahlnir said, due to housing interiors being instanced, the interior furnishings are only placed when you enter - when you leave the interior is 'deleted', so having you log back in outside the house if you happen to logout or enter another instance/duty while inside one is prevent you inadvertently zoning back into where a furnishing now is due to someone changing the furnishing arrangement while you were in the duty, resulting in you glitching into the furnishing and getting stuck.. or worse, crashing your game (this actually happened early on with housing, hence SE quickly implemented this change to fix it.).

Obviously this is only a potential issue for FC houses or personal houses with other players as tenancies, but personal chambers and apartments are also included in this process, probably just for sheer pragmatism. Unfortunately I don't see them changing this.