The answer is both simple and obvious. The same reason that many Germans left Fascist Germany - they don't agree with the politics and can't stand the culture.
One prime example of why Americans are bugging out is the Syria fiasco. The US has for many years started wars for no better reasons than commerce and hand-picking successor regimes. If other nations pulled half the crap that the US does in foreign countries, the people would be up in arms. Yet, they can't fathom why the world increasingly hates America, and why so many folks are getting the hell out.
Americans are so steeped in propaganda from the age of 1 that they just can't imagine why anyone would want to leave the country. They can't conceive of people not wanting government shoved into every orifice of their being around the clock. It doesn't compute that folks would disagree with a belligerent foreign policy. And surely no one would disagree with a centuries-long habit of killing large numbers of brown people.
Americans think that America is the greatest country on Earth. They can't separate the brilliant founding ideals of the nation from the actual practice. They think, "Hey, we invented the light bulb, the telephone and the automobile, not to mention landing on the Moon!" Doesn't occur to most of them that the last great achievement the nation had was almost 50 years ago.
Hello, America...wake up. G. W. Schrub made Edison's lamp illegal in favor of Frenchman Alexandre Edmond Becquerel's fluorescent lamps. Swedish and Japanese companies make the most popular telephones today. Detroit has declared bankruptcy and American cars are hardly made in the US anymore. And NASA no longer has a manned space program. The handful of astronauts it sends to the space station hitch rides with Russia - who could pull the plug on US manned access to space because of Syria. Furthermore, the US hasn't returned a man to the Moon since the early 70s.
Add to the severe economic decline of America its moral and ethical decay over the past 60 years. The US has gone from a position of moral and economic authority immediately following WW2 to a third-rate bully that ignores international law, violates the national sovereignty of nearly every other nation on Earth, and treats its own citizenry like criminals with an a priori assumption of guilt.
America no longer has anything to offer a person of conscience. The various levels of government in the US systematically violates every aspect of the individual's dignity and security. The citizen carries a massive tax burden to pay for unauthorized debt and unconstitutional wars. The police have gotten out of hand in addition to now being just one of a dozen agencies that all feel at liberty to violate human rights at will and with impunity. Finally, any attempt to redress grievances with the government are treated with derision and disdain by said government.
So, what exactly are the selling points for life in the US? Because from where I stand, there are none at all.
In fact, the current saber-rattling in Syria makes me all the more certain that I made the right choice to leave the US. I can not in any way support the military adventurism and the economic interventionism practiced by the US government. I can not abide the way that nation destroys families through an educational system and meddlesome agencies whose purposes are to undermine parental authority and transfer family loyalty to the state. Finally, I vehemently disagree with Washington's on-going policy of military interventionism on the flimsiest of justifications as a means of boosting profits for favored corporations.
The fact that Americans are asking such silly questions as, "Why Are Americans Leaving The US" shows the willful ignorance of the American populace. I say willful because the internet offers dozens on alternatives to the propaganda outlets at home. That they prefer CNN and Fox to real information shows the depth of the ignorance and/or the support that so many profess to their government.
Americans also display their corruption and complicity through repeated election of the same corrupt politicians, despite all evidence of the depravity of said politicians, based solely on being bought off by government pork projects. If this is not the ultimate cynical act of people who worship their own corruption, then nothing is.
If Americans had a real desire to change things, they could. There are 300 million citizens versus a few thousand minions. The only excuse for not changing things is willful participation in the moral decay of the nation, with the possible addition of laziness and greed as additional motives.
The exodus of citizens from America highlights the hopelessness of having ethics and integrity in a nation of ignorance and degeneracy. About the only position with a future in America is gang member, as recent reports demonstrate. Additionally, the only prop left under the American economy is invading, killing and destroying other nations. That is no basis for a healthy and viable future.
The intellectuals and business people who left Germany with the rise of Fascism were the only ones left to rebuild that country after the entire Earth massed to destroy it. Likewise, those folks bailing out of America today may end up being the only hope for rebuilding that country once the rest of the world has tired of its hubris. And that day may be coming soon.
The pretension of a gas attack in Syria as a way to justify (on the loosest of grounds) American military interventionism in yet another country may be the last straw. The world has grown weary of America's regime changes and death for democracy campaigns. The dominoes are decidedly lined up against the US should it decide to strike Syria. The first casualty in the war against the resurrection of Fascism will likely be Israel. The world is little fooled, at least any more, by the antics of the Zionists whose habit is to break a stick over its neighbors' heads, then run to hide behind Uncle Sugar.
The bully always gets his comeuppance and the US is no exception. The American people would do well to read the tea leaves of history to see what the future holds. A mass exodus of citizens is only one of many signs now visible.
Seventy years ago, it nearly destroyed our civilization to put one regime in a country the size of Texas back in its place. What will it take to contain the ambitions and decadence and the US?