Get More Enthusiasm for Your Judaism!
Because hashgocha on food is not an approbation for it; it merely defines the kosher criteria. People are given free will and can choose to eat something healthy or not. Now the conservative movement has gotten into more of the idea that you are expressing which they are calling a hechsher tzedek - I think. Suggesting that if a company were to get their hashgocha they would then meet certain standards - such as fair treatment of employees as well as humane treatment of animals.
When it comes down to it, there are different levels that people keep - like for instance chalav yisroel and chalav stam - no one can force another to do the opposite. People have to choose it - now, they most often choose based on their derech, but sometimes people make decisions based on different reasons.
Since you are aware of the lack of nutritional value in junk food, you can make the choice to not eat it and continue to gaurd your health. Tizku l'mitzvos! and l'chaim!