My opinion of this is that it has to come from movies and perhaps that portion of our mass media that is seen outside of the US.
I remember watching CNN in my hotel room in Munich the first time I traveled to Europe a few decades back. I was absolutely horrified. Seriously. I used to watch CNN almost exclusively here in the US. In case someone wants to come back with a Fox News comment, know that at the time FNN did not even exist (it came on air in 1996).
My first thought after watching CNN in my room in Europe was: Our enemies could not pay for better negative coverage of the US. I mean, it was horrible. Brutally negative. And very different from the CNN programming we were exposed to in the US. It made me realize why so many people had negative views of the US. Not the sole reason, of course, but this was quite shocking.
The other thing you have to realize in terms of the US is that this is far from a mono-cultural population, very far. There's tremendous cultural variety from coast to coast and north to south. And even within each region, cultures vary. I mean, just speaking of myself, while a US citizen, I come from both Latin American and Middle Eastern cultural backgrounds. By that I mean that I speak multiple languages, grew up in multiple cultures and feel at home in any of them.
And so, stereotyping Americans is, well, at the end of the day, fairly ignorant. I don't mean that as an insult but rather as a factual statement. Anyone who lumps "Americans" (as in the entire US population) under one category, culture or school of thought simply does not know what they are talking about.
Since we are talking about this, the term "American" sounds offensive (and ignorant) to a lot of people. How dare they call themselves "American"! That's a continent, not a country!
Well the name of the country is United States of America. In the case of Italy you can call yourself "Italian", France, "French", etc. What are you supposed to do with "United States of America"? Call yourself "United States-ian?". That's how you come to "American". This used to bother me until I understood where it came from and it made total sense.
BTW, there are other countries who do this kind of thing:
Mexico's official name is "Estados Unidos Mexicanos" or "United States of Mexico".
Brazil's official name is "República Federativa do Brasil" or "Federal Republic of Brazil"
In English we call them "Mexican" and "Brazilian" for short, just like people from the USA are called "American".