Squaring the Circle

Why do people talk about the 'four corners of the globe'? I'm prepared to accept, for amusement purposes, that the earth might be flat. But to believe that it is also square or rectangular is pushing it a bit. And if it's flat and square, could it not also be a cube? In which case we'd talk about the 'eight corners of the globe'. Or not.

Answers on a postcard please.