Answer by Jim Seidman:
In days of yore, most junk food was quite high in naturally occurring fats. When someone ordered a burger, french fries, and a milkshake, all of those were replete with fat. That meant that someone eating a junk food diet would experience a great deal of satiety after a meal, and likely only eat three meals a day.
In 1977, the United States Senate Select Committee on Nutrition and Human Needs issued what is widely known as the McGovern Report. Despite the objections of medical professionals, this report, written by a vegetarian aide of McGovern with no nutritional background, blamed saturated fat for a variety of health ills. The report recommended a reduction in consumption of animal fat. It also served McGovern's constituents back in South Dakota by encouraging much more consumption of grain, a major export of South Dakota.
Over the next couple of decades, junk food changed. Saturated fats were largely replaced by partially hydrogenated vegetable oils. Such oils are rich in trans fats, which are now known to stoke inflammation and lead to weight gain.
More recently, as the devastating effects of trans fats have become better understood, high-carb, low fat foods have become hugely popular. Unlike the junk food of old, foods that are predominately carbohydrates do not produce a lasting sense of satiety. People can drink soda and eat pretzels all day long, consuming far more calories than they ever could with the high-fat foods. These changes have been accelerated by farm policy, which has led to massive amounts of cheap corn that gets incorporated into junk foods. Cheetos made from corn meal, gummy bears made from corn syrup, and surprisingly many other products are cheap thanks to farm subsidies.
So, the short answer is that junk food has changed substantially in nature since the days of yore.