You wouldn't for multiple reasons. The fact that the tube you're covering is round is one of those reasons, and the primary one, but not all of the reasons. A r

You wouldn't for multiple reasons. The fact that the tube you're covering is round is one of those reasons, and the primary one, but not all of the reasons. A round cover would still be used if the tubes were square. A manhole cover has two purposes: people to traverse, but much more importantly, vehicles to traverse.