[mystified5016]

Radio buttons were also often round. The age of radio (and phenolics) was full of over-inflated round shapes.

But also, when you have a dozen monochromatic pixels to work with, 'square' and 'round' are pretty much the only usefully distinct shapes. Checkboxes were square for obvious reasons, so to distinguish a similar set of controls, you pretty much have to use a circle.

I'm pretty sure these concepts moved directly from physical systems to digital ones. Every person alive then knew what an empty square next to a line of text meant, and everyone understood the concept of ganged push-buttons. Just map it onto a pixel grid and you're good to go