Why don't cameras take round pictures?

Some might do - depends on what camera or settings they have.

There are some that can. While the lens produces a circular image, it was easier to fit square or rectangular images onto a strip of film. A 35mm lens used on a large format (5x4 inch or 8x10 inch) sheet film will indeed produce a circular image in the middle of the plate. It's a criminal waste of film, though.