The diameter of the circle is the same regardless of the point from which it is placed. It will not fall from any angle and will fall to the workers under construction.
And if the cover is square, it is dangerous because the diagonal length of the square is about 1.4 times the length of the side. If the angle is not careless, the cover will fall into the hole.
Another advantage of the round shape is that it can be easily carried by rolling. After all, a manhole cover is very heavy, and it can greatly improve work efficiency if it can be rolled.