Oh, that’s simple. It’s because the man-hole is round. Do I get a prize?
Some answers I get are:
Round items cannot fall into a round hole
They are heavy, and you can roll a cover
As cars go over them, they can rotate in the hole, and not corrode in place
Round is easy to manufacture
We (humans) are essentially round - so we fit in round holes
Round covers are immune to warping, like a rectangular cover
As they heat, they expand radially, thus eliminating stress fractures
So on, and so forth.