[hfdgiutdryg]

high calories, high salt, high fat

None of those are bad things (except salt if you have existing health conditions, and even then it may be a lack of potassium rather than an excess of salt). The single worst part of fast food is probably the sugar content. Mostly soda, but it's also added to nearly everything (a fast food bun I had the other day was ridiculously sweet).

Before WWII, the US was primarily an agricultural economy. People worked hard, without much automation, and food was comparatively expensive. IMO lack of physical exertion is a huge part of the problem, if only because of the discipline it builds and the reduced opportunities to snack.