Radio buttons are named after an actual radio. Mid-century car stereos had a row of chunky mechanical buttons for your preset stations, and they were wired with a lovely constraint: push one in and the previously pressed button physically popped back out. You could only ever have one selected. The mechanism made an invalid state — two stations at once — literally impossible.
When interface designers needed a control for “choose exactly one of these,” they borrowed the behaviour and the name. That’s why radio buttons are round (like the physical presets) and why picking one silently unpicks the last — the on-screen control inherited the car radio’s whole logic, down to the mutual exclusion.
It’s also why using them for a yes/no with only one option is a small crime: a lone radio button breaks the promise the shape makes. The shape says there are others.
The object died. The metaphor didn’t.