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.