You might be an MBBS, or an MD, or a DM, but you could still be quite illiterate; you might still not have the basic knowledge regarding the nutritional value of various food products. You could have a quack graduating even from the best medical college. So many doctors just do not know that milk or meat are nowhere close to being the best sources of protein, or vitamins, or iron, or calcium, anything. In fact, there is a lot of misinformation even on vitamin B12.
So, just as everybody else needs to be educated, the doctors too need to be educated because the public believes the doctors.