Putting aside the subjective viewpoint that such an idea is 'stupid'. The New World Order is not the same as World Government. The New World Order conspiracy is based upon the idea of a powerful, shadowy elite who control world politics and government from 'behind the scenes' so to speak.
World government is more of an assumed natural progression in human civilization. That eventually, as space travel becomes more commonplace, we will feel more united as a planet and our provincial squabbles will become more insignificant. I cannot claim to know this for sure, but I would presume a lot of Sci Fi takes ideas about future civilization structures and organisation from the Kardashev Scale which defines various levels of a civilizations technological advancement based upon energy production/consumption. Looking at the criteria, it stands to reason that the higher levels would demand cooperation on at least a planetary scale to achieve. Cooperation coordinated by a central planetary authority, in other words a world government.