Why should they be?
Well they are that way because the AAMC, ACGME, NRMP, and AHA got an antitrust exemption pushed through Congress. Therefore salaries are not determined in a free market.
In 2002, a group of three physicians led by Paul Jung, MD, a research fellow at the Johns Hopkins University, brought a class action suit on behalf of all current and former medical residents against a group of defendants that oversaw and participated in the match process and employed medical residents [2]. The size of the group of residents being represented by the suit was considerable, including all persons who had been employed as resident physicians since 1998 in programs that were accredited by the ACGME, as well as physicians in ACGME-accredited fellowships [1].
Those in the defendant class were categorized into two specific groups: “organizations and associations that participate[d] in the administration of graduate medical education in the United States” [3], e.g., the AAMC, NRMP, and ACGME, and “universities, medical schools, foundations, hospitals, health systems and medical centers that sponsor[ed] medical residency programs” [3].
Anticompetitive claims. The resident plaintiffs argued that the organizational and institutional defendants, through the match, had imposed anticompetitive restraints on medical residency placement and hiring by quashing the prospective residents’ ability to negotiate the terms of their employment contracts, resulting in fixed and depressed compensation packages [4]. The legal basis for the residents’ claim was that it violated Section 1 of the Sherman Antitrust Act, which holds that “Every contract…or conspiracy, in restraint of trade or commerce among the several States, or with foreign nations, is declared to be illegal” [5]. The Sherman Act is one of three core federal antitrust laws that regulate commerce in the United States, but some violations of the act may not necessarily be deemed illegal if the parties involved in the suspect activity can demonstrate that the restraint on trade is reasonable
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On April 8, 2004, Congress passed the Pension Funding Equity Act, and President George W. Bush signed it into law. The Section 207 amendment had two major legal effects. First, the provision confirmed that “it shall not be unlawful under the antitrust laws to sponsor, conduct, or participate in a graduate medical education residency matching program, or to agree to sponsor, conduct, or participate in such a program” [14]. Second, in relation to the first statement, the provision held that “evidence of any of the conduct described…shall not be admissible in Federal court to support any claim or action alleging a violation of antitrust laws”
http://journalofethics.ama-assn.org/2015/02/hlaw1-1502.html