[ryandvm]

If I know anything about capitalism, the primary reason manhole covers are round is because that's the most cost-efficient shape for the job.

[nonameiguess]

This is almost certainly the answer. They're round to fit the cylindrical holes they're covering, and those holes are round because they're made with drills, and they're made with drills because that's a lot faster and cheaper than digging.

[yobbo]

Tunnels are round because a round tube on the inside is stronger against compression than an equivalent square tube is. They are like vaults.