If they weren't round, superheros couldn't use them as frisbees/weapons.

If a car hits them and they bounce and spin, they will still (most likely) settle back into the hole correctly. Even a Reuleaux polygon won't always do that.

There are no points more highly stressed than others. Put an extreme amount of weight on them and the force is evenly distributed around the circumference, rather than overly stressing the corners and bending the cover (on a square lid).