Even if it was 99% "Whoever the non-Trump candidate is" votes, there's a psychological mechanism where, for many people, once they've made even an arbitrary choice, afterward they'll work themselves around to becoming enthusiastic for it after the fact. (Like many previously-wavering vaxxers who are now all-in.)

Where are the people who are after-the-fact Biden supporters? They voted for him, therefore he's now their guy and they fully and vocally support him. Where are they?