How come people think of America as only the U.S??

Ok, so EVERYDAY I hear people saying, "gosh, why are so many Mexicans coming to America...America is ours and Mexico is theirs" and stuff like that. I mean, can people be THAT ignorant, where they think America is only the U.S??? Geez! -_- Don't they even realize Mexico IS America too....Because AMERICA IS A CONTINENT, not a country....It annoys me sooo much everytime I hear someone say things like "Why can't Mexicans or Canadians leave America and go back to their home?"...People are sooo ugh! They seriously need a geography update if they think the U.S is the only America.......