Why is a world state implausible?

A world state is implausible because states need something to contrast themselves with. As Habermas says, in a line of thought borrowed directly from Carl Schmitt, the legal theorist whose exasperation with the constitutional arrangements of Weimar Germany led him eventually into the hands of the Nazis, ‘any political community that wants to understand itself as a democracy must at least distinguish between members and non-members.’ A world community of citizens, even if generating what Habermas calls ‘a form of democratically elected political representation’, would lack anything convincing to represent. It would have instead to fall back on ‘a legal-moral form of self-understanding’, which is another way of saying that it would depend almost entirely on the language of human rights. These rights can generate plausible legal procedures, they can generate widespread feelings of sympathy, they can even produce a general sense of indignation when they are violated. What they can’t provide is a sense of identity, of the kind that political parties think that it is worth fighting over.

from here