It's an artifact from evolving in a world where snap judgments were often the difference between life and death, and the person who assumed the rustle in bushes was just the wind, but it was actually a tiger, did not live to propagate. However, the paranoid person who thought it was a tiger every time the wind rustled the bushes, did survive to propagate.
Our current political system is pretty good evidence of people believing strongly enough in ideology to destroy the lives of others. I think they really do believe the stuff, and that's why it's so dangerous.
I have quite a few family and friends that unintentionally spread fake news because they really thought the story was real. One in particular is extremely hateful toward fake news but made a conclusion based on a headline that turned out wrong (we don't all have time to read every story we see, as much as we'd want to).
