Doctors get very little training in diet and lifestyle, because those are low-status and left to non-MD specialists, and many of them dogmatically cling to 1960s-era science.
Well, what can I say. No?
https://health.usnews.com/wellness/food/articles/2016-12-07/...
So yes, on paper you're right. In practice, there are even clinical nutrition specialists and those are MDs.
For example, a simple ketogenic diet intervention can be a very effective treatment for type 2 diabetes. But most doctors still favor the 1960s-era, discredited science saying dietary fat causes heart disease.
So diabetics are given metformin and whatever fancy new insulin came out recently. And then in 20 years, those people will likely lose their feet and eyes.
Drugs are initially optional, unless I am mistaken. However, you seem very optimistic regarding the ability of people to follow diets.