In this hypothetical, presuming you're putting the manhole (subsurface conduit/plumbing maintenance access point) in after you've already paved the road: it's because the tool you'd use to cut a hole into pavement (i.e. a concrete saw) cuts straight lines — and it's easier to make a square/rectangle out of straight lines than a circle. And sure enough, whenever you see workers hacking up the road, they generally are cutting square holes.

Refer, after that, to the process of constructing a manhole (https://www.envirodesignproducts.com/blogs/news/how-are-manh...).

At the end of this process, you have a square hole in the pavement, opening to a square excavation, bottoming out at a square concrete foundation, on which has been set a round concrete cylinder, which is then surrounded out to the edge of the square hole with packed earth.

Given this, you could equally-well finish this job either:

1. by placing a square of metal to fill the entire square packed-earth space you've constructed (as when bridging a pothole with a temporary steel surface plate);

2. or by first paving over the exposed packed-earth part, and then placing a circle of metal to cover only the manhole entrance itself.

...which is why people do justifiably ask why, in practice, we seem to always favor option 2 over option 1.

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.