Because being American is a nationality, not a geographic location. All those other countries may be in either North or South America, but none are associated with the name America as a nationality. They are Mexican, Honduran, or whatever. That is their nationality.
The U.S. of A is in N. America. But we're the only Americans by nationality. If the U.S. of A were located in S. America, we'd still be the only Americans.
That's like people who say that they are world citizens. We may all live on the planet Earth, but we're not all citizens of the planet Earth. Citizen denotes a type of political/national status. Americans are citizens of the U.S.A. Many other people are subjects, or nationals, or some other such title of their country.
My question is, why do people ask questions like that?
- 67GTO