That's really the kicker, I think. With the increased opportunities for networking and increased access to resources for academic support at top programs it makes sense that it would be easier for students at such institutions to achieve higher USMLE scores (and they would have had higher MCAT scores to get into said institutions) and thus more competitive/lucrative residency placements. But that's really just a population trend, and doesn't necessarily have to apply to individuals. If a student was able to succeed through standardized testing and make the proper connections while going to a mid-tier school, they too can achieve high scores and placement in competitive residencies, albeit the road was more difficult for them. The inverse could also be true, with high-achieving undergrads going to top programs only to fizzle out and not putting in the necessary work or taking advantage of their resources, and thus ending up with a low-paying position later in life.
Scores could be correlated with income, sure. But, to propose a correlation between academic institution and income would not work out very well, particularly with the already moderate correlation between various tests.