I guess it's more about intuition - they see when they damage someone immediately and stop doing that (e.g. wrong choice of prescription pills). With nutrition,

I guess it's more about intuition - they see when they damage someone immediately and stop doing that (e.g. wrong choice of prescription pills). With nutrition, they don't see the effect and/or don't attribute it to their advice because it takes decades to take effect, so they keep repeating the old advice. Practically all doctors I ever visited acted primarily on their intuition - "I have experience this works so use it".