I think the point is that a circle is not the only shape that can't fall into it's own hole/fitting, so it's the combination of this fact along with other facto

I think the point is that a circle is not the only shape that can't fall into it's own hole/fitting, so it's the combination of this fact along with other factors that make a circular cover popular. https://en.wikipedia.org/wiki/Manhole_cover#Circular https://en.wikipedia.org/wiki/Manhole_cover#Circular