Because both poles are reactionary, one is just more "civil" about it.

If the Democrats were willing to make a coalition with the left and actually fight against the fascists, then it would make sense to join them. But they are not. They are completely and utterly unwilling to make alliances or concessions to the left. They genuinely despise leftists even more than the fascists. If the fascists ramped up their killings and throwing leftists into concentration camps (something they have already been doing for decades), the Democrats will cheer on and provide the fascists with more funding, training and weapons (look at police spending under biden).

You can't partner with such "allies".