If you think "simply eating better and not eating so much" is "simple", you don't understand people. All our evidence is that it is by no means simple for most people. It's a problem that's about as hard to deal with as drug addiction.

Furthermore, it is a social problem, in that we as a society allow businesses to exploit peoples weaknesses without covering the social costs of the problems they are causing, and given that how people around you act strongly influence how able people are to stick to a diet.

To then act as if this is down to some imaginary free will is irresponsible.