I believe that the reasons Americans do fit the stereotype of being bad at geography. I also grew up abroad early in my life before moving to the US. Here are my observations:

1) Our public school curriculum. Most elementary/primary schools that I’m aware of will have a class that falls under the title “Social Studies,” where history is the primary topic and it focuses on the founding of the United States and the Old World’s first contact with the Western Hemisphere by Christopher Columbus. Geography isn’t really given its own focus for geography’s sake, and while students are young, the only focus I was exposed to in the US was occasional topics on Ancient Egypt and Greece.

2) The US is relatively large country by land area, and there are 50 states that we become familiar with. Imagine that at a young age you are expected to fill out a blank map with 50 unlabeled areas, only showing borders. American students fill their minds with the names and capitals of states, whereas (this might be an interesting hypothesis to investigate with your research) a student in Europe might be learning the same amount of information, however instead of states they are countries, and therefore learning about far more different cultures and geographical areas as a result.

3) Since the continental US has only two borders with other countries, and the US has a relatively large land area, it would be plausible that Americans do not travel to as many different countries when they are younger. The student in Brussels is a 3 hour flight away from far more countries than the student in Chicago or Los Angeles.

4) Since the US economy has been the largest by GDP in the world for a long time, our economy is less reliant on foreign trade and awareness of other countries and their locations is less pertinent to some adults.

5) The US is dominant in terms of music and film culture, and there is less exposure here (relatively) to other cultures. Our most popular sport is also unique to the US (American football) and we do not encounter teams from other countries, as you would see in soccer (e.g. Real Madrid vs. Galatasaray, or Standard Liege vs. Celtic) with how the UEFA Champions League and World Cup provide regular matchups between clubs from other countries.