The electors and alternates are already known, having been chosen at party conventions in each respective state. All set, an elector is on the ballot in each state. Now, the convenience of associating a presidential candidate's name with the party, and with the elector's name, is a real advantage. But is not legally required.
Presidential elections are very different, and the system tolerates surprize changes at any point in the process. Even after the November election, but before the electors cast their ballots in December. Even if the president elect is not available to be sworn in! And, even after swearing in. Agnew, then Nixon resigned, both due to scandal, and the result was Gerald Ford as president, before he ever ran for the office.