United States of America contains "America" in the name. That is why we call them "Americans", and their country is "America" or "The States", for short.
Calling Canadians or Mexicans or Argentinians...
You are right about calling germans eurasians is confusing, but so is the opposite. Saying the americans and the mexicans like mexicans are not americans is confusing too. They even tried to rename the gulf of Mexico to gulf of America like Mexico is not in America. I mean wtf... Yes it contains America in the name and this is what causes the confusion, so it is tricky.