So, why would we want this anyway? Uniting means that no-one will look at us from outside, and find our flaws. No reform can really come from inside a country or a system, it gets crushed from the top of the system, the folks who have interest in keeping the system going. The US only succeeded in its revolution thanks to the French, revolution in China doesn't succeed because no-one wants to cross the economic powerhouse that is China. I'm sure there are other examples, but I'm lazy, so fuagh.
Anyway, look at Orwell if you want to know why one-world government doesn't work. If you have no enemy, you make one. And if you make one within your system, it means you don't work with others in your group. Most of the truly successful times in countries are when those countries are at war against another country, and all put aside their differences to hate a common enemy. To paraphrase Voltaire "if (fill in name of opposing country here) did not exist, it would be necessary to create it".