C = M.D.
Medicine, though always seen as a rigorous field that only the smartest and strongest make it through, is actually a very formulaic process of which the hardest part is probably getting into medical school. A few often repeated axioms in medical school are: "What do you call someone who graduated last in their med school? Answer: A doctor" and "C = MD". Of all entering US medical students, 96% eventually graduate with a medical degree (in comparison, the same measure for graduate students in 62%). Most who fail to graduate, drop out of medical school for personal reasons, not because they were not qualified. Of those that graduate who want to practice medicine, all eventually find a residency program. With residency, it's a similar pattern. It is rare, but residents can be kicked out of their program. However, most find another residency program that is willing to take them. There are boards exams, but in many fields you don't need to be board certified to take care of patients, the pass rates are high and you can take them repeatedly until you pass. Of course, once you complete residency, even outright fraud or criminal negligence doesn't necessarily get your medical license taken away from you (I'm looking at you doctors who only serve as a prescription mill for narcotics).
The point here is that once you get into medical school, there is very little stopping you from eventually being a practicing physician. There are several problems. One is that there is a shortage of physicians out there and in many fields, such as family practice and internal medicine, there is a large number of open spots that need to be filled. A second problem is that medical schools and residencies thrive on their reputations. If their graduation rates decrease due to failure of a student or resident, it affects their statistics and makes the program less appealing to future applicants. Finally, there is a culture of avoiding confrontation in medicine. It's easier to pass someone than to deal with the trouble of remediation or removing the person from the program.
Please don't get me wrong. A vast majority of my coworkers and, likely, residents in other hospitals, are fine physicians who will serve their patient's well. However, the culture of passing everyone through the system, even if they are unqualified, has got to go. Patient's lives should be of primary concern.