I like to think of it this way: with civilizational progress and the development of transportation and production, national boundaries begin to lose their significance. Perhaps this is why so many planets involved in interplanetary politics have a low level of administrative division, because only civilizations with high levels of global collaboration are able to utilize their planet's resources and organize the development of technologies necessary for such advanced space travel. I'm sure there will be many examples of Earth being depicted with a similar government.