Rooms are rectangular because human brains find it simpler to map, I think. The order and structure involved means you can treat things much more interchangeabl

Rooms are rectangular because human brains find it simpler to map, I think. The order and structure involved means you can treat things much more interchangeably - for instance, the available directions for travel don't change as you move around in a rectangular grid, and rooms are all the same shape. The only reasonable alternative is a regular hexagonal tiling, and that means having more neighboring rooms, which gives its own problems.