A lot of healthy eating has significant time inputs, and there are a lot more things people prefer to do, like browse internet forums. A lot of people miss sleep, because they lose track of time online. There's a sort of "I just want to relax" after work attitude, and since exercise is work no one really want to do it. The big stigma around blue collar are that it's paid less than white collar. Also with increasing automation and the layoffs that accompany them, most young people are not interested in something that seems to be dying, with some notable exceptions. Last in is first out hits younger people especially.
The USDA food groups were also heavily manipulated, so grain was at the bottom instead of vegetables because the grain industry was larger.
Kale was a thing back in WW2 when the British government encouraged it's growth because it's easy to grow and nutritious. It's actually one of the many forms of cabbage and was heavily cultivated back in the Middle Ages.
There does seem to be this weird cultural idea that eating healthy is "feminine" and eating meat is "masculine", or "liberal" and "conservative". Effeminate liberals eat prissy weak weird shit like quinoa while strong red-blooded Americans eat red meat. There's also the belief that healthy food is disgusting, "It tastes bad therefore it'll good for you". There seem to be elements of self flaggelation involved in eating healthy. Who can suffer more, who can sacrifice more is better than their peers.
In conjunction with the PE thing, more water needs to be procipro. Water fountains are weak and inefficient in terms of water use. There's also very little opportunity to drink water through the day, unless you bring your own. Providing a water cooler and paper cups would help.
Passing PE is a requirement for some High School diplomas, but I don't know how wide spread it is. And as it's not helpful for going to college or getting a career, schools will ignore it.
Extending the school day by 2 hours cuts into extracurriculars. These are being pushed as deciding factors on who gets into a good school and who doesn't. You wanna pile them on to be a "balanced" student. Admittedly, many of these extracurricular activities will be sports, but a lot of people drop sports when they enter college or the work force to focus on their studies or career.