I can answer this one. America is simply at the fat, rich stage of its development as a society.
Hear me out. I'm only half serious but I think there is truth in this argument. The story goes like this:
You find a new continent that is not being used to its full potential so you colonise it and create a completely new society from scratch. People come to your "Land of Plenty" and for a couple of hundred years they work hard because they are motivated by building their new nation. Some find ways to tap unexploited riches in the land and they become an example to the rest who work hard for the "Dream." Huge wealth is created during this growth phase but it is not widely distributed.
Eventually people realise that the dream is pretty much exhausted so they look for other ways to catch up. They form trade unions and win more equitable conditions for the working class thereby creating a massive middle class and establishing a consumer age in the economy. Wealth is still created but it is widely distributed and increasing standards of living fund consumption. Wages rise so prices rise in a vicious cycle so consumer goods begin coming in from less developed countries that have cost advantages.
When you operate in a high cost environment, there are few ways to compete with low cost environments. You either work harder (for the same money) or you develop technological advantages that boost your efficiency. Neither of these work forever.
Your workers still feel rich but their standard of living gradually declines as they lose jobs or work harder and harder to keep up. They sacrifice leisure time, health and family to produce the efficiencies that they need to to enable them to continue to consume at their previous rate.
This is about where the US is at the moment. People are tired, fat, unhealthy and confused relative to 50 years ago.
I have no idea where the story goes from here but plenty of empires have risen and fallen throughout history. The difference is that it hasn't happened on a global scale, at least not in recorded history.
When it comes down to it, people only need food and shelter. All the rest is artificial. Most of our careers and industries and economic endeavour are simply providing luxuries that didn't exist a few generations ago. I can't see it all disappearing but I can't see it going on forever either.
Sorry for the long post.