Warning: the following paragraph contains a fallacy.
American culture is clearly getting dumber with time. The most popular TV news channel is Fox News, which likes to mislabel adulterous Republican senators as Democrats after they get caught with their mistresses, covers war like a football game and champions the cause to put the Ten Commandments in courthouses and prayer back in schools. The History Channel now prefers to run hyper-edited shows about UFO Hunters, Ice Road Truckers and toilet paper factories (under the heading "Modern Marvels", even). Should the television make you sigh and head for the bookstore, there too is a vacuum of sense: the computer-science sections are now depleted of all real CS texts and have been overrun with books like "Windows Calculator for Dummies" or "Blow your brains out with PHP in 12 days", and the science-fiction section has been completely replaced with vampire romance. Intellectual cinema is now banished to "limited release" markets, but the latest GI Joe movie has even more explosions per minute than the previous record holder "Transformers: Revenge of the Fallen". And there is truly nothing that makes Americans look stupid more than its cuisine, which now boasts such delicacies as Outback Steakhouse's "Aussie Fries", a single plate of which delivers over 2,800 calories. Food quality is now so bad that Vitamin Stores and GNCs have risen to sell mountains of pills next to such utter rubbish as Ginko Biloba and homeopathic remedies.
Reminder: the above paragraph contained a fallacy.
Where have all the smart people gone? Who's left that can count to 21 with their pants up? You might just as well ask why Einstein married his cousin, Elsa, and enjoyed eating sausage more than caviar. Why did Isaac Newton spent most of his life trying to convert lead into gold? Indeed, why don't smart people do only smart things? If everyone in the world was smart the culture of the day would still be kinda stupid because humans are only occasionally sentient.
The parts of American culture visible from the road are certainly suffering lately, but it's because you don't have to hit the road to find the good stuff anymore. Smart people are buying their books from Amazon.com now, so it leaves Barnes & Noble little choice but to remove the books that no longer sell and replace them with the ones that push political buttons or can pitch their value in 3 seconds.
Smart people don't want to get their news from television; they read a newspaper or the Internet. So Fox News caters to the people who do tune in, and because they're a 24-hour news channel most of their daytime viewers aren't employed and can spend a lot of time involved with their church.
Fast food is unhealthy because people no longer go out to eat healthy food; the notion of eating healthy is now closely connected with the notion of frugality and preparing meals at home.
But the most damning fact of all is that smart people are never smart all of the time, and they even have a name for it, the "guilty pleasure". Imagine if everybody in the world was smart, but 90% of the time they were on autopilot. They prepare four healthy dinners at home per week, but they grab fast-food for lunch and the other three nights. They buy Kubrick, Truffaut and Oliver Stone films, but they also bought tickets to see every Harry Potter and Star Wars movie because that's what all their friends were going to watch.
They bought a subscription to Nature, but they also surf 4Chan and ICanHasCheezburger.
Guilty pleasures are big business because everybody thinks they're smart, and therefore are allowed to indulge in dumb things when they want to.
If everyone was smart but behaved this way when they weren't working so hard to be smart, the world wouldn't that much different than it is now. There would be, perhaps, fewer books on how to blow your brains out with PHP.