I do not count calories and eat until I’m full. Yet I’m not obese, not even overweight. In fact, if I had to become obese, I’m not sure that I physically could. How does a theory of “personal responsibility” explain that?

Obesity is caused by some neurochemical fuckery that affects hunger and/or metabolism. That is a fact, supported by science, though the exact mechanisms are still very badly understood (in large part due to lack of funding for decades, caused by a completely misguided dogma that obesity is a moral failing). If it was all a willpower thing then how come some medications make people lose or gain significant weight?

(Yes, you can gain or lose weight by counting calories. However, every step of the way, you will be fighting your own body’s attempts to go back to its baseline, even if that baseline is very unhealthy. Of course in the absence of a better solution it’s better to lose weight by counting calories than staying unhealthy, but please realize that you’re in deep with the Dunning-Kruger effect and stop disparaging medical science).