Because it is...in addition to being an ethnic identity....
Seriously, in addition to my social security number, I have an enrollment number (issued by the tribe) and a CDIB card (issued by the federal government)
Anybody this day in age that is indigenous to the territory that encompasses the United States is also a citizen of a Tribal government...if they lack this tribal recognition, they are not native american in a legal sense, which usually is a good indicator of whether they are or are not acculterated into their tribal culture.
African americans on the other hand (such as yourself) fought long and hard for equal status with Whites. In the eyes of the law, you are just as much American Citizens as they. We, on the other hand, have been fighting for DISTINCTION. Some of us are dual citizens between the tribal government from which we come and the U.S. (like my children and I) and some choose instead to only be citizens of their tribal nation. So, in addition to not sharing common ancestry with other americans (the indicators of race) we also have the option of not being American citizens, which makes the legal definition of being a native american one of national recognition vs. ethnic.
Edit: we do not have special rights and priveleges, we have understandings, contracs, between the U.S. government and our individual tribal governments (there are over 500 of them).