Why do people oppose expansion and a world government?

This is just a question and I do not have a decided opinion on it as of yet, but I wonder some arguments against it. I don't really see the point in having almost 200 divided nations that bicker constantly over everything that you can think of. We are divided in thought, divided in direction, and divided in progress. There is a lot to work out to make an actual one world government effective, but it seems as if it could benefit humanity in the whole. You wouldn't likely have a risk of nuclear destruction of the world as they would be under the authority of checks and balances of one government, if anything separatist could get their hands on one or two but not enough to destroy the world like the thread we are on now. That is just one of many examples that would be beneficial. I will keep this short by ending with, United We Stand, Divided We Fall.