Why is it unlikely that any single political organization could speak for all aboriginal peoples?

Even aboriginals moved around and killed each other when they felt like it. Why are we romanticizing the Stone Age?

Because they each have different concerns. Sioux Indians want the Treaty of Cheyenne followed by the USA. It gave them the entire western part of South Dakota including the Black Hills. The Hawaiians want their islands back. But some want a bunch of money instead of their land back. So since they have different desires, no one group can speak for all of them.