I think to a large extent it’s a case of cognitive dissonance.

Loads of these people have defended Trump for years, supported him despite his obvious lies and grifts and so you kinda have to believe that whatever else he comes up with is also true. If you believe all his previous falsehoods, why not the next?

To admit he’s full of shit means your whole belief system has to change. Trump supporters have lost friends, alienated family, spent their money on him… It’s much easier to keep believing in him than it is to admit you’ve been wrong all this time, cause that would mean having to admit you’ve been taken for a fool this whole time AND it means all your efforts and sacrifices have been for nothing.