pffft, what a silly question. Manhole covers are round because manholes are round...why would you make a square cover for a round hole? (Seriously though there

pffft, what a silly question. Manhole covers are round because manholes are round...why would you make a square cover for a round hole? (Seriously though there's a lot of conceivable reasons, and while that's one I doubt it's a major one, a lip prevents it. I think the fact that a circle doesn't need to be "lined up" and that they are easier to move, etc are much more important)