> Calories-in, calories out works perfectly for everybody else (including obese people before they get obese). [...] Willpower certainly is the problem for most people.

It just does not. If you focus on that only, it leads to diet that makes you feel like crap all the time. It makes you low energy, so you struggle or flat cant maintain performance and precision in work. It makes you irritable and it makes it much harder for you to handle normal stresses - meaning your kids and wife and coworkers basically suffer.

And what it does is that people eventually decide it is not worth it.