Most people won't adhere to a diet that would make them healthy.
Stupidity? Yes, stupidity on a very deep selfish gene level.
It's NOT that they don't understand the price they are paying.
It's that they make choices that the genes are making: not making waves, not bringing attention to themselves, not being different, fitting in is more important than being healthy, having energy, feeling good, calm, being intelligent.
Almost everything "social" is gene directed. I say "almost" because I am humble. I don't know any social that is NOT gene directed, but there could be, maybe.