The other thing for me, I think, is the circle matches the shape of a person's face. Obviously I don't think the circle is a face, but it's easier to quickly re

The other thing for me, I think, is the circle matches the shape of a person's face. Obviously I don't think the circle is a face, but it's easier to quickly resolve a circle inside a square, than a circle inside a circle. (Don't get me wrong, it does look nicer when round.)