In the USA as in other countries the brightest graduates are attracted into a limited number of professions, such as medicine, law, and management, because of the disproportionately high salaries in those professions compared to others.
The earnings of leading professionals cannot be justified merely by invoking principles of market economy, nor by suggesting that their high salaries are necessary to induce people to enter arduous professions. Given the number that are unable to gain entrance to academic institutions for such professions, the earnings in such professions are clearly higher than a free market would allow.