Why there are no Indian who live in the big city of US today?

I will take the AP US History next year, so I need someone who can answer this question for me. From what I know, I know there are Africans, Europeans and Native Americans who live in America since 400 years ago. I know that Indians had been here since the Ice Age, and European bought slaves from Africa to America. My question is: why are no Indian who live in the big city of US today? Why are they only occupy less than 5% of US population while African Americans and White occupy a large amount of population in US today? Are they still live in the reservation today? I am not try to be a racist here, I just want to know about the history, sorry if I offense someone. Can someone please answer the question for me. Thank you!!!