From the Creator who put them here. They inherited it. Since the beginning of time. Answers to these questions have been around far longer than the questions themselves.
Tribal sovereignty flows through American history in a timeless river, without beginning or end.
The reality here is that tribes have always been sovereign, a fact recognized in the actions and laws of early European explorers, a fact recognized as exploration became settlement, and a fact recognized as settlement evolved through colonial and into national government. That means it existed since time immemorial, and is recognized as such in the Constitution of the United States. States and tribes have equal legal and constitutional status in their dealings with the federal government. Most commonly known of the government-to government relationships between the United States and Indian Nations is the power of Congress to make treaties. The relationship extends to existing reservations, some created by Congress and others by Executive Order of the President.