Probably because when most houses were implemented, the Tibia Store didn't exist and CipSoft wasn't too concerned with this type of customization.
Maybe someday they will change this, in the same way they did with the beds (in the past it was not possible to remove them or move them). Or maybe not, since this change may be more complex than it appears to be, and require a lot of time and resources that they prefer to spend on other things they consider more important.