It always bothered me that in a lot of popular science fiction, there is one government, one civilization, one culture, and one language per planet if said planet is not Earth.

That is totally unrealistic and unbelievable.

Instead of making one totally fleshed out civilization, I would make enough to make that world seem diverse, that were as fleshed out as their number of appearances and the nature of their roles in the story calls for.

Additionally, if people on Earth can't unite and live in total peace and harmony under one government, why would people on other worlds be any closer to doing so? This is of course assuming people everywhere have the same sort of basic nature.