Or it could be the things in the hundreds of papers from before 2005, when normalizing obesity was not politically expedient. All you have to do is live in any east Asian society for a period of time to understand that portion control and shame are extremely effective in preventing obesity in a population.

First generation Asian immigrants to the west generally do not become obese. They retain the ideas from their home country that being fat is shameful, and they retain their conception of a correct portion size. If they start getting even 3 kilos overweight, they take steps to undo that. The west has decided portions are to be huge, and is in the process now of destroying shame. Time has shown the results.

Unfortunately, doctors cannot do anything about widespread societal shift in attitudes toward health. All they can do is recommend what is most likely to actually help. While the correct answer is "stop eating so much; gather the willpower to be mildly hungry during the day; your entire mental conception of healthy food and portions has been fucked since birth," people won't actually listen to that. Thus, modern medical advice has been forced to basically admit that no one will ever lose weight in aggregate and begin making recommendations with that in mind. That doesn't make the older mindset incorrect, though. If a doctor could mind control you into being optimally healthy, there would be a lot less focus on "it's a disease; it's stress; you can't help it so just try to manage it best you can" and a lot more "stop constantly eating so many calories".