Basically doctors are poorly trained on drugs. They have a short course on them once in college.

They are predominately trained in observation, anatomy and labeling of illness. They then consult therapeutic manuals, many of them way out of date (because they are expensive to buy), on how to treat what they have just diagnosed.
For example methylcobalamin and oral supplements of B12 are typically not in their manuals, and it never occurs to them to look elsewhere for newer information.

I read an interesting critique of medical school in US recently that likened medical training to a cult indoctrination. I can't find that exact article but this one is similar:
http://www.naturalnews.com/021922_ju..._medicine.html

If you Google Is medicine a cult? You will find more.