The human body is bewilderingly complex that it it still a subject of thousands of researches worldwide. How can you simulate something you don't completely understand?
Plus, in the case of medicine, one needs to know also how to interact with a patient. Is he/she telling the truth about his/her symptoms? Is there something the person's not telling that might affect your diagnosis? What about educating the patient about the disease?
These are real day-to-day situations that can never be simulated.