Because there are no angles for alignment, the round shape makes these heavy covers easier to put back on once they're taken off. Round covers are also easier to manufacture. But the main reason manhole covers are round is so they won't accidentally fall into the subterranean abyss. With a round cover, no matter how you hold it, you can't shove it in. It just won't go. If it were square, a prankster could hold the cover diagonally over the hole and drop it in, to be followed by who knows how many scooters and pedestrians.