The burden is on the patient to be their own advocate and parse through the various opinions (and their own research) to decide on the best treatment choices, with the assistance and guidance of the experts. Because despite them being experts, they have insufficient time to research and understand all of factors that are applicable to an individual's situation. (For example, in at least one matter, I proposed a significant change to the treatment plan and my oncologist agreed with the underlying logic and we made the change. Had I not mentioned it, it wouldn't have happened.)
It seems like there are a few bounds within which a doctor, and the healthcare system (including insurance companies), are prepared/allowed to vary treatment. I imagine some patients, especially those with potentially terminal conditions, are interested in treatment options that fall outside of those bounds.
(Perhaps if one's cancer follows one of the most normal/common paths, then blindly following standard of care is sufficiently wise. I'm not sure how often this is the case, but my impression is that there is great variation even within cancers of the same general type.)
I'm sorry but there are cases where they are definitely guessing and will sometimes admit as much. It's an educated guess but if you ask a doc to, for example, strongly justify why they selected one of the three approved PARP inhibitors for an ovarian cancer therapy, you're going to see a little of that uncertainty in living color.