Why are the covers of sewers mostly round?

The roundness of the sewer cover is mainly determined by the characteristics of the circle. Since any diameter of the round cover is not less than the circle where it is located, the cover of the sewer will not fall off the wellhead anyway. Any side of the square cover is shorter than the diagonal of the square opening where it is placed, so when the cover is upright, it may fall obliquely. Therefore, in order to prevent the cover of the sewer from falling off, people design the wellhead and cover of the sewer into a circle.