Giggle. And, tell the interviewer there are probably a lot of reasons. You may mention that a round manhole cover allows people to work a little bit more quickly because they do not need to line up a shape to put the cover back on; since it's round, it will fit no matter how it is set back into place. A round manhole cover also will not fall through the hole. From a safety perspective, a shape with corners could be a hazard as people could cut/poke themselves on it. The round shape also makes the object easy to roll which means less chance for injury because people do not need to carry the object.
"Design aesthetics of course! (Just kidding!) A round manhole cover cannot fall through its circular openings, whereas a square cover can, if rotated. Circular covers also do not need to be rotated or aligned to close them up. Round covers are also easy to move or roll around."