Because the other question is being used as justification, I'll call upon the Power of TVTropes as well.
One World Order:
According to a good deal of Speculative Fiction set in The Future™, it is the natural order of things that all governments will merge together to create a central authority to govern the entire species. It's not necessarily the human species, however.
Sapient aliens also almost always have a single government to whom every law-abiding sophont in their race answers. Any conflict between members of the same species will be called a civil war. Especially true if The Verse of the show contains boatloads of sapient species.
That said, I'll also copy what I put in the comments and expand on it a little. It's not quite as simple as you make it out to be...
- The Narn from Babylon 5 have several religions, and are open to atheism. A new religion on their planet was even created accidentally during the course of the show, with a new religious figure who hates the role he's ended up in.
- The Vulcans and Romulans both came from the same planet, and had vastly differing religious/cultural views. But even after the split, before the Kir'Shara was found, there were multiple religious sects on Vulcan with different interpretations of Surak's teachings. The most prominent was the Syrranites, perhaps that name sounds familiar...
- The Klingons had their strange offshoots as well, such as the group that headed off to the Delta quadrant in search of their savior. They encountered Voyager in 7x14, Prophecy.
Now, there's one thing I'm sure you'll notice from these, that would seem to support your claim: They (almost) all still hang on a core belief system from that culture. The Vulcans believed in Surak, but split off, the Klingons in Kahless, and the Narn started with G'Lan, adding more prophets through the years.
This is very familiar to most of us in western culture: This is what happened to Judaism, with one branch splitting off to become Christianty, and then it happening again with Christianity to create Mormonism.
Write what you know, eh?