TMI = Too Much Information

The answer is: Efficiency.

Sewer piping is round (either circular or oval in cross-section) because it minimizes friction of the inner pipe wall with the moving fluid. With only gravity - that is, change in elevation - to drive flow, maintaining the velocity of the liquid is critical to carrying the, um, solid components. Those solids would also accumulate in the corners of a square sewer. Yuk.

In the trade, "manholes" are the underground structures through which access is gained to the system; they're cylindrical, too. They have two purposes, the more important of which is as points of intersection of two or more flows, and/or change of direction in flow - thus, they're round for the same reason pipes are.

With the rest of the system round, circular access "manways" (the trade jargon for what you see set in the pavement) only make sense. Not only is a circle the optimum shape, saving material and weight, but it has no preferred orientation - neither for installation or for entrance through it.

Finally [and probably the true answer] -- a square or rectangular cover has a chance of being dropped through its frame by a clumsy sewer department worker (or a vandal), leaving 50 or 60 lbs of cast iron at the bottom of the manhole and a 36" diameter hole for someone to fall into. Circular covers can't fall past their circular frames.

That was a rhetorical question you asked, wasn't it?