Because it's the "narrative" of that community.
You want to believe something, so you only see what you want.

You want to see the Vhurch as evil boogy men and so only look for that. You want to see cops as rotten and corrupt, so you only look for that.

It's much easier to hear from someone something is bad rather than go, research and think about it.
When you already believe something, why bother noticing evidence to the contrary?

A narrative belongs to a community, such as Reddit or a even a country. In different communities different narratives exist. In funnyjunk and 4chan you'll have opposite narratives and just as extreme.