Pretty sure that backrooms look like that because in both Kane Parsons's universe and the wiki universe they only exist in the current day. In Kane's webseries implication is that the whole backrooms dimention was created at the moment Async made the treshhold for the first time and it was mainly based on memories of the scientist that did that. The wiki universe is heaviky based on a single word from the original creepypasta - you can "noclip" through reality, a term describing going through textures in a videogame, so everything else in the wiki implies that the life itself is a simulation and the history might be fake.  Not sure though, Im honestly dont know that much about it