Why don't more people realize that Chronic diseases can be prevented by diet?

I live on a mostly raw organic high-quality Vegan diet and I never get ill in any sense, so I for one feel that a high-quality Vegan diet/lifestyle can do wonders. Over the past 30 years, incidents of type II Diabetes has tripled. Heart disease and type II Diabetes are almost always the result of an unhealthy lifestyle. It has been said that these diseases can be prevented and reversed through a very healthy diet and physical activity. Also many Cancers can be prevented in this way too. Why don't more people realize the health and longevity effects of diet and lifestyle instead of slowly killing themselves on processed chemical and fat ridden food? Or do they just not care?