A lot of the medical research is focussed on finding cures for (serious) diseases. This is mainly due to funding issues. The lack of focus on health as oppossed to illness is a major factor in health care costs. A fitter population will be more productive and will lead to less people becoming ill, making the burden of healthcare costs a lot less.
E.g., most of the reseach involving vitamin D looks at its possible role in diseases; But, this totally ignores the possibility that an optimal level exists for which recuperating from exercise is the best. People at that level will find it easier to combine a demanding job with regular exercise. This will lower the chance of illness, even if vitamin D itself has no direct role in the illnesses prevented by exercise.