Annoyed 🙄😡

I just need to vent. My best friend is totally American, and of Vietnamese descent. I'm white and of Scandinavian descent; we're both 3rd generation Americans. Yesterday we were at a festival for the Fourth of July, and man in front of a us in line for food asked my friend how long she had been in America and whether it was her first Fourth of July. She politely replied she has lived here all her life, along with her parents who were also born here. The man wasn't even embarrassed, he just looked surprised and said "that's why your English is so good!" This pissed me off. Why do people assume that anyone other than whites must be immigrants?! He didn't ask me where I was from? Or seem surprised by my good English? Why does American automatically mean white to so many people?? Ugh annoying.