Well shafts are mostly round, so the manhole cover is made round.
Many wells are round, so the manhole cover is naturally round. So why are the wells mostly circular? Because in architecture and civil engineering, circular channels are the most conducive to maintaining soil pressure.

The force of the circle is more uniform and not easy to crack and collapse
After the circular well cover is stressed, the pressure will spread to the surroundings, and it is not easy to crack and collapse due to the uniform spread.