Actually "primitive" people in reallife are anything but not civilized. They often have very high moral standards, very interesting religion and lore, etc.
For good reason "barbarians" who get a taste of "civilization" prefer to return to their "barbarism" instead.
Just an example, primitive people spent like 20 hours per week doing "work". Which means doing things like hunting, fishing, collecting fruits. In many regards the things we "civilized" people do during holidays.
D&D Barbarians are inspired by various sources, both historical and ficitional. In effect they are simply a different approach to the pure, magic-less warrior, than Fighters.
Thus D&D Barbarians can exist in any context. No need for a tribal origin of the character. And Tribes can just as well produce Fighters, too.
