[Hodar]

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.

[Constitutions Grandchild]

Other than my answer, the only two I might entertain are the last two, but they beg the question. If you asked why MAN-HOLES are round, that's another subject altogether. The only answer I could come up with is that pipes are round and man-holes connect to pipes and are not prone to having debris lodge in corners.