Because medical knowledge is so complicated, the information possessed by the physician as to the consequences and possibilities of treatment is necessarily very much greater than that of the patient, or at least so it is believed by both parties. Further, both parties are aware of this informational inequality, and their relation is colored by this knowledge
And, as Deborah Haas-Wilson notes, the information asymmetry is also two-sided in many cases. What does the patient know that the provider doesn't? Likelihood of medication and treatment adherence, their own medical histories, their individual preferences on trade offs involving treatment and side effects and more.