Is it because it's a university course first and foremost and therefore all the uni mitigations and accomodations and allowances apply equally to a course in medicine as they do to English lit?
I'm not involved in education, so can't answer about how mitigations apply. However, one of the issues is that academic performance isn't necessarily a predictor who will struggle as a doctor. You don't actually need to be that academic to be a doctor and half (at least!) of what you learn in med school is of no day to day relevance to your work.
So someone can do brilliantly academically, and still be entirely unsuited to be a doctor. Conversely, many people who fail exams at med school will go on to be excellent doctors. Really, the weeding out should be of people with the wrong personality traits or lack of emotional resilience but - of course - that's incredibly hard. And who would you pick in practice? A lot of the doctors who struggle most are high-achieving, highly motivated women who will be excellent doctors to their patients, but who will go under emotionally because they cannot cope with the mismatch between patient need and what they are able to offer.