The reason is because of the complexities. We still do not fully understand how the brain functions, there are still mysteries associated with bio-chemical, hormonal, immune-system, DNA and other physiology. Couple that with mysteries that are yet to be answered in differing responses of people with the same physical makeup, same disease and same treatments. Human physiology is vastly complex.
Physicians are diagnosticians, believe it or not, some diseases are still being discovered and classified. They can provide therapy for people who may not fall neatly into a diagnostic criteria but are suffering. They are detectives and follow clues, they are parent-like in making recommendations to adults in living healthy life-styles and can assist them in accomplishing that.
So while there may come a time when technology is able to reduce the number of needed clinicians it is still a very, very far way off.