Charlie:
Yes, many opiates are prescription drugs. In fact, the active chemical in marijuana, THC, is a prescription drug. Any drug will be a prescription drug if it is a controlled substance that the FDA determines to have medical benefit.
So marijuana the drug does have medical benefit in a pill form, and is available under prescription, and is regulated by the FDA. But, smoking it (as opposed to taking it in pills) does not have have any identifiable medical benefit according to the FDA. And that is why you cannot get it by prescription.
I thought you believed in science and I was the lunatic who thinks science can be political