I don't think we need to wonder. Wide variety of tasty, often cheap, processed, high-calorie food. Americans making more money to eat in restaurants more often where they are generally optimizing for taste, not health. More sedentary lifestyle, both at work and in free time. All kinds of couch-based entertainment available, not the least of which is your phone, why go outside? And finally, probably because of our online malaise, people do seem worse at tackling challenges. Take all the griping there is now about Daylight Saving Time.