To your point 4, we have lots of good reasons why people are gaining weight: they eat more. It's not exactly rocket science.

Obvious reasons include:

a) food is very cheap, especially processed food

b) portion sizes are comically large in the US

c) ubiquity of snacking and eating on the go, which is mostly high calorie

d) few people smoke now, and cigarettes are potent appetite suppressants

e) less social disapproval of being fat. To fair degree, being overweight is contagious

I agree completely with your last paragraph, at a society level. At an individual level, though, it's all on us to exercise self control for ourselves. Just most people don't want to.