Because they don't engage in good faith. Right and 'left' fascists don't value truth as anything more than a tool.

That's why it's so easy for them to switch from (say) genocide denial on the issue of China and its genocide of populations in Xinjiang, Tibet, Outer Mongolia, and throughout the (Artificial and Imperialist) borders of the state.

Because they will often come into a conversation knowing full well that what's happening is a genocide. But since their objective isn't to reach truth, their argumentation doesn't reflect the things they know.

It's one of the recurring traits of Fascism- Disdain for reality. The way this trait is shared by reactionary authoritarians like those who run the Chinese state is quite telling about the nature of these 'left' authoritarian ideologies.

The 'left' is in quotes here because these groups are something-phobic, socially reactionary and abusive as often as not.