[Mad Dawgg]

Because sewer pipes are round so that sewage doesn’t collect in the corners

[MaxMax]

You can’t use logic when dealing with Dem supporters. Any answer is wrong if you’re on the wrong side.

[steve86]

Because sewer pipes are round so that sewage doesn’t collect in the corners

I think you're right about the sewer pipes but wrong about the access port (also circular-section pipes are strongest against compression forces). The manhole cover is round, as several have explained, so the cover can't fall in as a rectangular/square cover could. . Other than that the shape of the vertical access does not have to match the horizontal pipe. Why would it? Below the manhole cover the access is not necessarily cylindrical.

Lots of shapes out there

I guarantee the passage beneath the triangular cover isn't triangular.