What does a teacher do in a school? A teacher would tell you off or give you lines or whatever it is, detentions, but at the same times, they are wanting your best interests at heart. They are teaching you, they are educating you, but at the same time, they will also have the ability to sanction you.
In medicine, there's a fairly large but still finite body of knowledge that you need at hand for most of your daily work. It takes a few years to learn it, but once it's there, it's there. With writing, on the other hand, every new book - indeed, every new story - is a fresh and terrifying reinvention of everything.