I've done med school ordered assessments for students having troubles for a few years. The modal student with problems is one that was marginal on some stat to begin with (e,g, low MCAT scores) and scores in the average range on IQ testing. They may have impeccable undergraduate records, 3.98 GPAs from top schools in molecular biology, whatever, but when they get into the bigger pond of medical school, they are just teetering over the edge with respect to bombing out. These are students that worked really, really hard, harder than their peers throughout middle, high, and undergraduate work. But, when they get to medical school, everyone works hard. They've lost their only advantage and they fall. They might make it given a perfect environment (e.g., your 20 something with no family pressures or other issues), but throw a little adversity in the mix, and they are in big trouble.
Of course there are other scenarios. Some students are brilliant, but mentally ill. Some never wanted to be there in the first place (pressured by family to become a physician) and aren't expending enough effort. The rare student has some sort of learning disability combined with really high skills in other domains that allowed them to get to this level.
In psychology, we take those students (and below) and put them in professional schools.
That last comment is taking it too far. The professional schools definitely take those people, as you're saying. However, the majority of professional school students are above that level in terms of ability. Furthermore, I've known plenty of hack clinicians coming out of solid university-based PhD programs. They might do fine in a research-oriented career, but I cringe at the thought that some of them are free to work with patients if they choose. This isn't due to "intrinsic" issues in most cases--it's the way they were trained in their program.