Not much of a revelation, just an observation, but how much of sci-fi pushes the ideal that Humanity's path forward is unification?
Star Trek is a major one, and many would state it was a progressive show, and it earnestly was.
It also stated that in its ideal future, there's a Federation of like-minded extraterrestrials, and as a member species, Humanity lives in a united, post-scarcity society under a single government.
To that end, how many alien species in these stories are so inhuman, in that they have no differences, a world-wide government, a single language?
It's laughable how direct the propaganda was, and continues to be.
As far as Earth having a single government, I think there's this idea that what you might call "political identity" scales with the "known world". For example, medieval people didn't no much about the world and thought of themselves as being from this village, barony, county, etc. In modern times, we think on a more national scale. In a time where we might know people from other stars, one would assume we'd think of ourselves on a planetary scale.
This may or may not actually be the case, but there's a method to the madness.
And yet the village, the town, the nation have not disappeared. Because local government will always be more sensitive to the needs of the local people than any distant and disconnected authority. And it doesn't really matter if that authority is on the other side of a planet or the other side of a galaxy because neither of them are here.