For starters, there's a gugleplex of medical information out there, daily there are new studies, and as a result we have need of countless specialists.
Also, everyone's body is similar, but not identical. The same treatment (a standard treatment) might do one person a great deal of good, while it might destroy another.
Further, diagnosis is complex. Diagnosis on the human body is not like a controlled experiment, in which the doctor holds the variables constant.
In addition to this, patients lie (it's a huge problem doctors face daily - the omission of information), forget to give important details of medical history, and/or are not compliant.
Insurance companies look at the bottom line, but not only with regard to patient/clients. They also trick physicians to avoid paying them. For example, they are held to a limited amount of time within which to make claims for payment. If they fail to submit claims within this window of time, they are denied payment. If the wrong code is provided, they are denied payment. Doctors have to hire all kinds of personnel at great cost in order to get paid by insurance companies. This is all part of the technique used by insurance companies to avoid paying doctors.
Insurance companies also charge doctors huge premiums for malpractice insurance, premiums far larger than they need to charge for the amounts that insurance are having to pay out.
Insurance companies are making out like bandits.
Medical students know all this and no longer wish to pursue being general practitioners and internists. We already have a shortage of GPs and internists.
It's hard to be a doctor nowadays.
We need a system of health care like that in England.