1. BMI was basically just pulled out of a hat, and it turns out that for many health concerns, the overweight class does better than the normal weight class, with the actual “clearly worse” outcomes really coming up as you trend into obesity.
2. Environmental and genetic factors are huge contributors to people being overweight. To some degree moralizing about weight loss is a bit like telling kids with asthma they should live in places with less pollution. Or telling someone with breast cancer to not have the BRCA gene.
3. Doctors would regularly miss health problems because they’d blame them on people being overweight / not exercising enough. Especially for difficult to diagnose issues, it’s easy to just point at the number you have that “looks bad”.
4. We still have no firm reason why people are gaining weight. The popular answer is sugar and processed foods but those very much already existed at scale in the 50s.
The tragedy is that it turned into a morality play where fat people were blamed for it, despite in many ways being as genetic as getting breast cancer.
The reality is people who actually study obesity know that basically GLP-1s are the only health intervention other than bariatric surgery that consistently works. At a population level no intervention has reversed fattening anywhere in the world.