There could be a million other variables, but they are not all equally plausible. As for the ones you identified, I'm not sure they could be reasonably measured: how does one know that one enjoys practicing medicine without actually having practiced medicine? This seems like a catch-22. Is there a correlation between number of shadowing hours and physician competence?
I don't recognize a difference between being "well suited" to something and being "good" at it, nor is time spent obtaining a license proof of anything. Available statistics indicate that American medical care is, on average, unexceptional and unproductive. Given that physicians are at the top of the healthcare hierarchy and that they enjoy a relatively high degree of autonomy in the U.S. (by virtue of not being government employees), I can only assume they are either incompetent or crooks. Healthcare spending is going up at a completely unsustainable rate, so we need to figure out which it is and devise some possible solutions (modifying admissions policies might be necessary but it would obviously not be sufficient).