Louis Menand describes The Ph.D. Problem:
It is easy to see how the modern academic discipline reproduces all the salient features of the professionalized occupation. It is a self-governing and largely closed community of practitioners who have an almost absolute power to determine the standards for entry, promotion, and dismissal in their fields. The discipline relies on the principle of disinterestedness, according to which the production of new knowledge is regulated by measuring it against existing scholarship through a process of peer review, rather than by the extent to which it meets the needs of interests external to the field. The history department does not ask the mayor or the alumni or the physics department who is qualified to be a history professor. The academic credential is non-transferable (as every Ph.D. looking for work outside the academy quickly learns). And disciplines encourage — in fact, they more or less require — a high degree of specialization. The return to the disciplines for this method of organizing themselves is social authority: the product is guaranteed by the expertise the system is designed to create. Incompetent practitioners are not admitted to practice, and incompetent scholarship is not disseminated.Since it is the system that ratifies the product — ipso facto, no one outside the community of experts is qualified to rate the value of the work produced within it — the most important function of the system is not the production of knowledge. It is the reproduction of the system. To put it another way, the most important function of the system, both for purposes of its continued survival and for purposes of controlling the market for its products, is the production of the producers. The academic disciplines effectively monopolize (or attempt to monopolize) the production of knowledge in their fields, and they monopolize the production of knowledge producers as well. This is why, for example, you cannot take a course in the law (apart from legal history) outside a law school. In fact, law schools urge applicants to major in areas outside the law. They say that this makes lawyers well-rounded, but it also helps to ensure that future lawyers will be trained only by other lawyers. It helps lawyers retain a monopoly on knowledge of the law.
Weirdly, the less social authority a profession enjoys, the more restrictive the barriers to entry and the more rigid the process of producing new producers tend to become. You can become a lawyer in three years, an M.D. in four years, and an M.D.-Ph.D. in six years, but the median time to a doctoral degree in the humanities disciplines is nine years. And the more self-limiting the profession, the harder it is to acquire the credential and enter into practice, and the tighter the identification between the individual practitioner and the discipline.