They do treat the cause. If an illness is being caused by a germ or bacteria, an antibiotic that kills those germs does deal with the cause.
Now, there are some diseases that have "non-medical" causes. Things like stress, poor diet, being overweight, etc can cause diseases. But explain to me how your doctor is supposed to "cure" you of stress? Or get you to eat a proper diet. They can not follow you around 24x7 and control what you put in your mouth or how stressed your lifestyle is. That is an unrealistic expectation.
Western Medicine does deal with the physical cause of disease. It is not their job to deal with the "spiritual" cause. That is the job of religion, which western culture is more and more rejecting.
While eastern culture seems to think they can cure anything with meditation and "inner peace."
The truth lies in between the two extremes.