The question is, why do we still need nation states, if the whole world consists of non-barbarian insiders?
This was never what the state was for. The state is the engine of civilization, it made the civilized rather than the other way around. It is the source of education, it is the spring of economic advancement (see merchantalism or the american school of economics), it is the mechanism by which the interests of one state are put over another. In short the reason the state still exists in a few places around the word is because it is exactly what makes progress.