1. Because USMLE scores are confidential (your school only has the right to know if you passed).
2. Using USMLE scores as a screening/selection tool is ostensibly an abuse of the USMLE. The test is a "minimum competency test", and to actively make available the numbers would put the USMLE Secretariat into the same camp as those specialties that use the scores as they do. The USMLE (as far as the NBME et al. are concerned), Step I, II, and III are simply and exclusively for initial medical licensure.
If the USMLE gave up the game and gave out all the numbers, that would indicate that they were in (tacit or outright) support of using Step scores for residency, and not exclusively for licensure. For the USMLE Secretariat to drop off the high road, and do that, would give ammunition to the proponents of making all the Steps pass/fail. By not supporting using scores like they are now, the USMLE is able to turn a blind eye to the misuse, and don't have to face the 'pass/fail' arguments.
This differs from the MCAT, because the MCAT is designed to be a stratifying device - to find the (de jure or de facto) "best and the brightest", whereas the USMLE supports the "what do you call the guy finishing last in his class? Doctor!" idea.
So, in a nutshell, re: USMLE scores: the USMLE isn't giving up the numbers, and so, you would have to poll every single resident, or every single residency director (provided the students have released the results to the program), who would give an average under risk of being sued for millions for breach of confidentiality.
So you ain't getting it. Sorry.
