A narrative has been formed for them that fits nicely into their perception of themselves, and conveniently gives them an enemy to focus all their hatred on. It's really easy, they don't even need to think. The story that they believe is their opinion is confirmed everywhere. Mainstream news, government, late night comedy, social media, search engines, and many of the people in their surroundings who have been sold the same narrative. Even more convenient, all the news or opinions countering it has been removed.