I think a few of the posts reminiscing are getting closer to the root, it isn't a pure diet/exercise issue but rather a broad lifestyle/behavioral deterioration.
The rec center I workout at is a two story establishment and the weights/weight machines are split among the two stories due to space, the amount of able-bodied folks I see using the elevator to go between machines is frankly disgusting, especially when the stairs are faster. Even 20 years ago you never would have seen something like this, the elevator was for the elderly or handicapped. This is among a cohort of people who have made the decision to go to the gym to improve their health, behaviors like this are even more pervasive and commonplace among the general population.
All this to say, sedentary is becoming more "sedentary" than it used to be. The idea of doing a flight of stairs or parking an extra 60 feet away from the grocery store in the parking lot (also ends up being faster and safer when you factor in the additional traffic closer to the front of the store) has become so burdensome to some people. Convenience for convenience sake at the expense of normal/healthy behavior.