The Universe is hard-wired to replicate existing objects and spaces. Instead of Noclipping through reality you noclip into another timeline. These Timelines have mostly been destroyed except for tiny slivers or reality the universe copies these locations and replicates them in mass creating the maze known as the backrooms. This explains why someone can say 1 trillion people are currently in the backrooms, they come from different timelines.
The reason entities can be found here is because they are races of universe destroying monsters from the hive. This also explains exits, they are gateways used by the monsters.