It isn't as simple as you say, because a tribe is more than you seem to think it is. I can change my country, I cannot change my tribe.
Having many tribes does not mean unity is impossible.
You're missing my point. There are no tribes in Australia, for practical purposes. Australia has moved past the concept of tribes. In Australia, people are born into Australia.
In Africa, in 2017, people are born into a tribe.
That is utter, utter crap.
The Tiwi don't associate with the Kimberly kids. Their islands are closed to outsiders. You are given your totem at age of 12 in a smoking ceremony that outsiders are not permitted to witness. You are not born Australian. You cannot marry one. You are Tiwi.