Why do healthy people (athletes, vegetarians, etc) who eat nutritious foods and exercise regularly still get heart attacks?

Multiple causes: Great question! a healthy diet & exercise significantly reduces risk of heart attack but not entirely. Young athletes who die suddenly usually have genetic/congenital causes. There are many genetic factors in heart disease. Inflammation, heavy metals & other environmental toxins, infection & stress are common causative factors. Diet & exercise may not be enough to always counteract these.