Because leftists and liberals have inherently opposed ideologies that cannot agree on the basic way to oppose fascists. A core axom of most leftist thought is that fascism is the end stage of liberalism. It's the ideological equivalent of cows and chickens working together at a Chick-fil-A

This narrative stifles dissent from anyone who disagrees with you. The Democratic platform attempts to reconcile a large tent of competing views. So we should not assume that criticism of "our side" is evidence of ulterior motives. We all need to demand more when we see our government failing so spectacularly.

I just ask them: Show me the demolished deathcamp, then I can see if you really are antifa.