The problem with houses is that the only thing they'd really accomplish is removing players from the "outside game world", as they sit in their houses - which is the exact opposite of what I'm trying to achieve. Quest hubs and villages exist to bring people together, and everyone being in their house, away from the game world, kind of works against that.
With that in mind, it would be a rather big change to implement with little (if any, at all) benefit for the game, really. There is the map editor, which you can use to design your own map - but even that is barely used.
On the other hand, this has been implemented. Houses are also accessible by others, though house owners may lock them.