Well, if Biden and Trump are both fascists, I think many people would still argue that Biden is the less dangerous fascist. It really doesn't change their main reason for voting for him, and that is that he is the least bad option.
I've said for years now that the Democrats and the Republicans are like a good cop, bad cop team. They both might be on the same side, and they both might be against you, but I think it's understandable why people would prefer to deal with the good cop. It's a harm reduction measure. The bad cop will almost certainly do more harm than the good cop.