You want to get to the root cause IMO its convenience and value.
In most cases we have a longer work week and a we all commute, this puts tremendous strain on the individual to maximize time spent.
McDonalds will sell you a big ol meal for $6.00 without having to leave your car.
Everything in America is mass marketed and produced, so companies invest tons of money into making their food taste better than their competitors, last longer on the shelves and come in higher quantities. The "taste better" part is where for a long time things went awry, as this means add more sugar and more fat. Soon all the cheap easy to cook food is un-healthy and real tasty. This drives out the market for expensive and healthy food. Hummus which is just ground chick peas costs 5 bucks for a little container. I could buy 2 pints of ice cream for that.
Even in college a chicken ceaser salad with one piece of cheap quality chicken/lettuce cost me $4.50, where as a bacon cheese burger with fries and a soda cost only $4.25
So in short: No time for exercise/to cook and the cheap food is the worst for you. It's not necessarily the American citizens fault but more a result of the size of the nation and its economic structure.
You see all this health obsession generated by our massive Pharmaceuticals industry which is now back lashing into food...Once the media hype gets large enough companies start offering "light" version of all their products and trying to profit that way, but we've all been eating the high fat/sugar versions for years and that stuff is addicting.
Let me end all this by saying that I'm big guy and I've been bigger and smaller than I am now, I'm 5'8 and never been below 198, a lot of this is metabolism and my lifestyle. I was at 265 for awhile and right now I'm around 225. The times I did the best where in college where I had free time to hike and had to walk everywhere I went. In the working world with a desk job it's real hard, I don't drink any sugar/fat, just water and diet snapple. I try to eat healthy but I'm limited to what I have the patience to prepare/what is served at the cafe at work. One can only eat so many turkey sandwiches in a week. Even the salad bar no has more noodle salad and cheese in it then vegetables, and salad dressing has more fat in it than the hamburger half the time.
I'm not a morning person and when I get home from work at 7:00 all I want to do is watch TV, it's hard enough to find time for bass. Forget it if I have band practice/bass lessons and I don't get in till 9:30 or midnight after work. I pretty much have to eat on the run or not eat/eat right before bed.
BTW my grocery list = baked chips, baked nachos/salsa, 2 packages of lunch meat (turkey and ham), bread, 4 cans of tuna(in water not oil), eggs, pickles, no sugar jello, 5 green apples, a container of hummus, crackers, cereal bars/cereal (basic 4), diet soda, white rice/chicken, sometimes some almonds...and I usually tack on a block of expensive cheese + crackers. Usually comes out to just over $100 bucks, and it varies a little, sometimes I eat baked beans, or a lot of soups...all the healthiest stuff I can find with the exception that nothing but the apples can go bad quickly, because I don't have time to grocery shop and if I buy lettuce, or mushrooms or w/e it all goes bad.