Because voting is a multi-step process often involving more than one action. I have experienced all the below votes at governmental level, they are not hypothetical to me:

Sometimes the voter has the ability to add arbitrary options to the ballot.

Sometimes multiple values are allowed.

Sometimes there are positional votings.

Sometimes there is ranked-choice.

Sometimes you do not even want to submit the vote.

Looking only at "these are the options available, choose one and only one" is not a valid strategy for reaching the conclusion on what UI idiom to use.

I protest against the default assumption that radio buttons (no matter how much I like them) should be the default go-to for voting interfaces because of the context of voting is too complex.