It’s possible that some of them don’t want to admit to a stranger that they’d change their vote.

But, most of it is probably choice-supportive bias or post-purchase rationalization. They believe they’re smart, right-thinking people and that they couldn’t have made a horrible mistake, so they look for reasons to feel like they made the right choice, despite all the evidence against it. They’re supported by a whole group of propaganda networks that explain away every bad thing that Trump does and shift the blame to democrats, liberals, trans people, immigrants, non-whites, women, etc.