It’s not that you are voting for a party ticket, instead you are voting for electors picked by the party, with the only job of supporting the candidate. (In my state, the ballots even say we are voting for “Electors For …” .) And in some states, once the electors are chosen on that ballot, the law says they must vote for their pledged candidate. So it’s not easy for a candidate to just drop out. Those electors may not be given the choice to change.