Because evidence doesn’t matter to his base. They don’t believe anything negative about him, all of it is lies. Likewise they believe everything bad about his enemies.
Their beliefs are not rational. They are based entirely on emotion, not facts or reason. And they hold those beliefs so strongly that they identify as part of themselves. That belief cannot be shaken, because it means that part of them was wrong as well, and that couldn’t possibly be right. They would have to actually self-reflect, and they’re incapable of that.