There isn't a wrong answer to this question unless you are asking an engineer about a specific manhole cover they have data about.
> Manhole covers are round because manholes are round.
That's clearly not the whole story. There is no reason why a shaft has to be covered by the same shape as the shaft. If there were significant drawbacks to having round manhole covers, we'd just have another shape with dimensions large enough to not fall down. There aren't, in fact round covers are fairly easy to manufacture as long as high precision isn't required, as simcop2387 noted.
There is a strong reason for the shaft to be covered by the cover of the same shape: it requires less material than any other shape. So with all else being equal, it's the optimal shape; it makes sense to use other shape only if there are other outweighing concerns.
There isn't a wrong answer to this question unless you are asking an engineer about a specific manhole cover they have data about.
> Manhole covers are round because manholes are round.
That's clearly not the whole story. There is no reason why a shaft has to be covered by the same shape as the shaft. If there were significant drawbacks to having round manhole covers, we'd just have another shape with dimensions large enough to not fall down. There aren't, in fact round covers are fairly easy to manufacture as long as high precision isn't required, as simcop2387 noted.