I am not a doctor myself, but from what I hear from relatives, who are, for most known diseases, there is a predefined treatment plan. While the doctor can choose something else, he better knows what he is doing, otherwise it will be legally dangerous. Even if there is a well-known better treatment he isn't allowed to do it, because it isn't approved by the council of insurance, and this will also result in legal trouble. If the doctor doesn't work in a hospital that won't only result in legal trouble, but a revoked profession, i.e. the doctor is out of business and bankrupt for life, because the insurance decides where a doctor has a praxis.
If the case arises that the disease it is not known and it is not known by another doctor in a higher hospital, then you do need to select a medication yourself. And then you do need to know both the physiological issue and the mechanism of action. Not just to ensure that the medication is effective, but because there are thousands if not millions of medications and you do need to select. Simply choosing something will almost certainly result in serious harm/death. Also, because it is an unusual disease, the mechanism of action won't be the primary/intended one of the medication. I mean, not that the doctor wouldn't never be wrong. But he will always /think/ he knows how they work, because if he doesn't he would rather do nothing (meaning only treating pain / trying to prevent it becoming worse) than something, for legal reasons.
That's why the studies are famously only about learning by rote, because of the huge knowledge that must be known by heart. Also it is more important to know the physiology, because there will be constantly new medications and you need to understand their mechanism. This is also why in this profession you are required to go to continuing education or otherwise loose your profession.
I think for most mistreatments it's not because the doctor doesn't understand the medication, but because of wrong diagnosis, focusing on the wrong symptom, misunderstanding or simply human error. Every conclusion any man takes, can also be taken by every doctor, but in addition he also has the vast knowledge and training of decades and the century long history of the medical community. That's why I disagree with you and agree to the original claim:
> thinking that our own judgement is better than a doctor judgement, supported by a vast community and shared knowledge, is epistemically interesting.