Medical students and nurses do take diet and nutrition courses. I'm in a prereq nutrition class right now...
Even before being diagnosed with a disease there are huge returns to regularly exercising and staying fit. You feel better, look better, have more energy, sleep better, the list continues. A doctor saying, "you need to lose weight and exercise, it could help with your diabetes," is a relatively small motivator compared with all of the other benefits.
https://www.hsph.harvard.edu/news/hsph-in-the-news/doctors-n...