> The real answer is because manhole covers are mass produced and manufacturing circular ones is usually easier and cheaper than the alternatives.
My understanding is the primary reason is because circular covers cannot fall into the hole itself, whereas that problem exists with other shapes like squares.
Equilateral triangles can fall in. You have three dimensions: the height of the triangle is less than the side length of the triangle, so it fits in near the sides of the hole.
Which includes circles and Reuleaux triangles, which are much more difficult to manufacture than circles. I think this can be rounded off to "circles are the only well-known shape that can't fall into a similarly-sized hole".
My understanding is the primary reason is because circular covers cannot fall into the hole itself, whereas that problem exists with other shapes like squares.