I'm not forgetting anything. Those are all permanently loaded and available zones, that fill a fixed and accounted for space on the World servers. Even apartments and FC quarters are limited. (they also don't take up as much space as large houses or external wards) And the inn room is the same model for all players, just like places like the Solar in the Rising Stones/Waking sands, it's essentially a shared zone even if you don't see anyone else there, so it requires far less live data transfer.
To make housing 'instanced' like that for everyone, they need to allow for unlimited instances, which is only possible via the Instance servers, which is what requires a duty queue.
Basically, the cost for having housing "instances" as they currently exist without a duty queue (they're not 'instances' in the games technical vocabulary, they're just zones) is that they are limited in supply.