Blaming things on another group makes that group feel so much more superior, dominant, and gives themselves a boastful, delusional feeling of, "Wow, I'm better than everyone else on the planet" feeling. Which helps in the long run, when the other group actually does something wrong, then they're feelings of hatred will then be "justified". That can lead to feelings of a nationalism, which can give a country great strength in their time of need.
"That which can be asserted without evidence, can be dismissed without evidence." -Christopher Hitchens- My Hero
