I can try to answer that for you.
Maybe no one on the right believes the cases to be legit. I mean, how many people have 2 Misdemeanors that were out of the Statute of Limitations elevated to 34 Felonies? Really? No citizen not named Trump would have ever had that happen to them. If someone can provide a cite, I will stand corrected.
And let's look at the classified documents case. Trump had them and Biden had them. Regardless of how each handled the case with authorities, the actual crime is the same.
Did they have documents they were not supposed to have? Yes.
Is it a crime? Yes!
Were they treated in a similar fashion? No!
And the public sees that. And in some ways Biden having them is worse since he was NEVER authorized to have them from when he was a Senator. Possibly not as VP, but certainly not as a Senator. So either he skated because 1) he was the President or 2) as Hur noted, he wasn't fit to stand trial. Neither is a good look.