The way the game is designed relies on a lack of levitation and jumping spells. They needed to make everything run on console (and designed the UI entirely around that, it’s torment to use vanilla UI on PC). Cities have to be in separate worldspaces, and levitating could get you into the placeholder spots on the map. It would also make closing those Oblivion gates trivial if you just float up to the top of the tower. (Real men just use paintbrushes).

It just doesn’t make any lore sense though. Necromancy is illegal too, but it’s certainly still happening!