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.
lol at point e.
This is so not true. For a while there was a tiny blip where a corner of the internet was fat friendly.
I know a black lesbian from the south who was obese and managed to lose fat by weightlifting. She said by far being obese was worse socially than either of the other things. As part of getting in shape she started jogging and people mocked her for being fat while exercising.
This claim is utterly silly on its face. Look at pictures of folks in the US 60 years ago compared to today. The population did not randomly mutate a genetic condition towards the propensity of obesity. Folks who come from societies that are not all that overweight did not magically develop a genetic condition upon immigrating to the US. This claim is entirely non-credulous and most folks claiming such are doing so in bad faith.
Environmental is by far and away the largest factor. Meaning most everyone lives surrounded by highly palatable horrible-for-you food and lives extremely sedentary lifestyles entirely optimized for convenience.
It's really not something that needs ANY more examination than that. Fixing it is the hard bit. Everything else on top of this is a rounding error and not something that needs to even be discussed until the first order factor is fixed. Then we can start discussing nuance, outliers, and details. It's a waste of time otherwise.
GLP-1s are the only thing that has turned the tide even a little bit. Because people eat less. Any other benefit from that is again - secondary and perhaps interesting, but not the primary mode of action. GLP-1s proved it is the American diet that is the problem.