What's With The Unified Government?

Star Trek - one semi-utopian human government. Firefly/Serenity - one over reaching government that has varying levels of influence on different planets. Battlestar Galactica - one human government both before and after the attack. Dune - one government, although a much looser structure than the other cases - will come back to that. The Foundation Series - one government. Hitchhiker's Guide To The Galaxy - only one human. Star Wars - one government (not counting the rebels as a government), in 1-3 arguably still one government, although a bit more UN like. Futurama is the perfect example - in 2000, pre-serious space exploration, there is the current set of countries, in 3000 when there is lots of space travel there is one Earth/human government. There are a bunch where government doesn't really play a role and the basic assumption is there is only one exploring space: Rendezvous with Rama, Alien, Dark Star, Red Dwarf, 2001 A Space Odessey, Ender's Game, Total Recall, iRobot (the book).

There's plenty of science fiction not involving space that involves multiple governments and some that involve their interaction (ex: most Neal Stephenson books). There's plenty of science fiction where aliens come to Earth and there are multiple governments (War of the Worlds, Independence Day, Close Encounters of the Third Kind, ET, Transformers), although I'm having trouble thinking of any where it really matters that there are multiple governments. There's some science fiction with multiple governments while there has been some movement into space, but the story is almost entirely about Earth (ex: Blade Runner/Do Androids Dream of Electric Sheep). So why can't I think of any stories where humans have multiple governments exploring space? Do they exist and I just can't think of them? Is there some assumption that if people are going into space either everyone has unified or there is a single space bound country? Is it just avoiding the complexity that would be added to stories? Am I just not going back far enough - did people get it out of their system during the space race?

I think that we will get to serious space travel/colonization before the world will unify under one government. There is the UN, but I think it is far weaker than any of the governments listed above (if we found aliens would it really be the UN that would negotiate with them?). Plus it seems like it would open a lot of possible stories if instead of the humans trying to deal with say the Romulans, the US was trying to create a treaty while the EU was at war with them and at the same time China and the US were at war (BSG is sort of touching on this right now, but they split the cylons instead of splitting the humans). Or even if there are no aliens, just something a bit more complex than one big oppressive government and one small rebel group. Of all the examples I listed, Dune has the least centralized government and some of the story comes from the maneuvering of the groups that make up the government and how the main character plays them off each other. That further makes me think there is potential there.

Also science fiction can be helpful when it predicts something and works through some of the issues ahead of time (such as iRobot and some of the old images of space ships). Since there's an increasing number of countries capable of space exploration it seems like a topic worth exploring.

Ok, I should really stop writing now since it is quite likely someone will have a list of a bunch of stories I'm just not thinking of.