I also used to assume it was a field full of highly intelligent people, but when I told this to my cousin who is a doctor he laughed and said something to the effect of "The medical profession selects for mediocrity", and went on to point out that being a doctor is a fairly rote profession; most of their work does not involve creative problem solving or critical thinking. It is a very challenging profession in terms of having to learn an enormous body of material, work grueling hours in dangerous and unpleasant environments, and deal with customers who are definitionally unhappy to be your customer. But it's a set of challenges that a fairly wide portion of the population can rise to.

It's practically 100%. Once you're in, they'll do everything in their power to graduate you (legitly). Every incentive they have points at a perfect graduation rate.

EDIT: In the US, anyway. Things are different in, say, Caribbean medical schools.

I read someone mention that French medical schools are easier to get into but the graduation rate is much more like engineering schools. About 1/2 to 2/3 of students drop out or fail. Said the result was French doctors were functionally better in general than US ones and happier.

Since I don't have any experience with French medical schools I don't know if that is true or still true.

I do have experience with US doctors and about half of them are just phoning it in every day.