Corrupt admins will already do that, so giving blanket immunity doesn't actually help anyone. All they need to then corruptly prosecute anyone is a court to agree that some act was "unofficial".
It has never happened before now. It isn't because past presidents have never been criminals, it was convention. It's a way for your country not to turn into Haiti or Zimbabwe.
Nixon was very probably criminal, but Ford’s pardon prevented a prosecution without having to settle the question of whether Nixon would have been immune in the absence of a pardon.
