They aren't even all round in the US. Where I live there is a high and rapidly growing number of data centers. In fact there is currently another large data center campus being built for the company famous for asking this question. Among other things this means installation of fiber optic cabling along the roads leading to the data centers. This in turn means the installation of a large number of manholes to service the cables. All the covers for these manholes are rectangular.

Manhole covers come in a variety of shapes. A non-ironic "why are manhole covers round" question speaks more to lack of life and/or travel experience than anything else.

There are triangular[1] and even more exotic-shaped[2] covers.

[1] https://www.reddit.com/r/mildlyinteresting/comments/g826ve/m...

[2] https://manhole.co.il/doSearch.asp?tp=114

And look at that, it's got a hinge on it, so it can't fall in. And it can't be stolen or misplaced.