Harder to fall in the hole itself. Vs a square or rectangle cover

Hard to put a round cover on the wrong way. Only 1 way to put it on, even if upside down

People are generally cylindrical in shape, hence a round hole to go into.

Because they have to cover a round opening

Because there is fiber cable that tunnel between buildings and having manhole allows people to pull new cable but these manhole also need security which is why if you pull the cover off an alarm will go off tell someone that's it has been open. Also there is normal a second lock as well before you can get into most manholes.

good question, could be because squares are more expensive and use more material since a circular shape would have less surface area, also maybe easier to create in factories.

I'd imagine you can get it to the hole easier by rolling it on edge, it wouldn't chafe/snag the wires pulling things down the hole and it's harder for a vehicle to snag a corner or sharp edge driving over it. But what do I know ... Lol

Nobody will get hurt going down the drain.

Really?

Who cares?