Why are microwaves rectangular?

Most microwaves nowadays have those automatic revolving thingies in the middle that help ensure even nuking of your food. I've never disabled the turntable on my microwave, and I've never witnessed someone who did. So why are microwaves rectangular? You can't put something in there that's longer than the short side of the rectangle or else it'll get caught up when the turntable goes around. Why not just make them square-shaped? Your thoughts are welcome...