Why One World Government Will Never Happen

One item I've been repeatedly told by various conspiracy theorists is that there's a big conspiracy to combine all countries into one very large one that covers all territory on earth. This nation would unify the legal codes and currencies, and resistance would be futile. Mostly because it would be impossible to move away from it.
It's...not going to happen. For a number of reasons.
For one, governments literally can't just impose whatever dictate they feel like. And I can't think of an issue that this government could take a position on without driving some part of the world into riotous anger, with pitchforks and foaming mouthes and looting and burning. Economics? The kind that would please, say, India, would send the US's central regions into a fit. And vice versa. Religion? Everyone wants theirs to be the official one, and all the others banned. And no, no one cares about the massive contradiction this would involve. A few tolerant people are willing to live side by side, but there's not nearly enough of them.
Even currency will involve massive conflicts. Countries mostly choose strong currency or weak currency strategies based on what kind of industries they have. If we have only one country on Earth, we clearly can only chose one. Regions that have the "wrong" suiting on this will be infuriated.
But the biggest reason that this would fail is that we have no commonalities. Most countries have a shared identity, be it ethnic or ideological. And we humans can only, at the neurological level, comprehend the existence of about 150 people. Attempting the circumvent this leads to stereotyping, racism, and other means of "lumping" people you have little to do with into one person. So to try to "lump" 6, almost 7, billion people together, when they have nothing in common and will spend the entire time stereotyping each other, and all hell would break loose.
Lastly, the benefits to this are questionable. A unified government might have slightly increased trade, maybe, if it somehow managed to retain order, which it probably wouldn't. Mostly, it'd be like the current German - Greek crisis, to the billionth power, and with way more guns and pitchforks. And every country on Earth is, to a degree, in debt. A one world government would assume every last one of those.
So for pathetic benefits and massive drawbacks, this is worth basically no one's while, so it'll never happen.