Why do so many country's perceive americans as dumb?

I have lived all over europe, and have traveled to many other country's, I can tell you that it is common for people in other country's to perceive americans as dumb, lots of these country's have comedy shows on tv where they go to the USA and ask people on the street questions about things like 9/11 and very basic common knowledge questions. they take the dumbest answers they got and put them on the show, the thing is though, why is it always america that they do this in? the films Borat and Bruno are also an example, they were filmed in the US because it was believed people were dumb enough to fall for his stunts and believe he was a real person. Some americans can be quite hurt by this, its horrid.