I'm American and the first time I remember hearing the term "American exceptionalism" was when I was about 30 years old. Even then, I've almost always heard it brought up in the context of questioning it. Maybe once or twice I've heard a politician on TV professing to believe in it. To this day I'm still not sure I know what the phrase is supposed to mean.

So I don't think it's as widespread as you say, though I can easily imagine that it's a popular idea in certain segments of the population.