People from the United States of America are called (in English) "Americans" for about the same reason that people from Estados Unidos Mexicanos are called "Mex

People from the United States of America are called (in English) "Americans" for about the same reason that people from Estados Unidos Mexicanos are called "Mexicans"--that is the most suitable general designation for those people that is distinctive from how people from other countries are designated. The usage in Spanish is a little bit more complicated, but there isn't much confusion about how to write English on this point.