Someone give a2ng0d a hearing aid, he's not listening.
We can't unite all nations under a single flag for the many many reasons that the rest of us have described.
Plus, we have another thing to deal with, along with what has been described: Laws.
The Middle East, there are laws against Homosexuality, laws against women driving, speaking to males other than her husband, and it's okay to stone someone to death. In the U.S.A, you can't have booze in public, use drugs, smoke or look at porn till you're 18, drink till you are 21. Canada, you can use weed, drink at 16 (I believe). In Japan, Lolicon (hentai or pornographic (All drawn) imaging depicting underaged children in sexual situations) doesn't bother anyone. In China, it's okay to eat domesticated animals like dogs.
Everyone has different laws for different things, and only laws of murder, rape, fruad, theft, arson, public chaos and destruction, those are universal, but the punishments aren't. So who's law do you follow?