Undergrad/premed: live with family, have 100% 24/7 familial support of your education, living at home with all of your essential basic living needs taken care of.

Residency: move to a different location away from family, no longer living in a dormitory environment with the expectations associated with being a student but are now a real adult making your way in the world. Suddenly you have to make the whole package work on your own without laundry/cooking/mental health/financial support.

Now you can no longer put 100% of yourself into your studies, but instead can only manage the 60 or 70% that most people can muster when they have to actually maintain their physical existence while also meeting their professional expectations.

It happens. Often incompetence is specific to one specialty - neurosurgery is competitive, so you can assume that anyone who gets it has at least adequate grades/test scores. But that doesn't mean that they're clinically worth a damn.

I'm an anesthesiologist. There are people who wash out because they just don't have the temperament for it. They're not dumb, they're not even bad doctors, they just aren't mentally equipped to sit back and relax while running a code.