I don’t know if there is a good answer to this except “history” but I wonder why あなた ended up being the word for calling someone in the 2nd person in modern Japanese, when if you think about こそあど, it should be そなた, as was common in classical Japanese. あなた should refer to someone who is not near the speaker or the listener, if you think about it in the context of こそあど.
I’ve never heard そなた or こなた used, though the words existed / exist. Obviously どなた remains a polite way to refer to an unknown party.
I’d be interested in reading more about the whole history of pronouns.