Left wing sites cannot exist without heavy moderation or censorship in the same way that cults cannot exist without policing people's speech.

Sites without constant intervention end up turning right wing - I believe this is because left wing sites kick these customers off.

Society needs natural 'arguments' and 'debate' and 'ridicule' to keep itself balanced. Historically we were not able to 'ship everybody we disagree with' to a separate island. This creates the echo chambers and radicalization that we see today.