Humans evolved to allow hunger to override the rational thinking part of the brain. The older parts of the brain can basically shut off the newer (logical thinking) parts. So even if you know you have to eat less to lose weight, often if food is available you won't be able to resist it if you are really hungry. Your statement is wrong, it is more complicated than that because we are hardwired to not starve if food is available.

And yet no country in the world, rich or poor, Western or non-Western, managed to reduce its obesity levels to, say, three quarters of the maximal value or less.

So it seems to be, in practice, a more complicated problem than reduction of smoking, which has been rather successful in many countries.