They refuse to look at the evidence for whatever reason
Not even so much refusing, it's just intellectual laziness. There are droves of Americans who know close to nothing about most things of relevance in the world. But ask them about the latest contestant on the bachelor, or the latest hollywood dating scandal, they can rattle on for hours.
Sorry to say, but I travel to Europe every year and in my dealings with people there, I'm always amazed at how cultured, and knowledgeable people are over there.
The stereotype of the American idiot, who is arrogant, ignorant, prone to excesses is to some extent true. Not always, not everybody, but enough to justify it.