According to Law Professor Eugene Volokh, a leading reason law schools have only one exam per course is, that's the way law students want it:
Most law school exams, to my knowledge, involve a single end-of-semester exam. Some comments in this thread argue that this is pedagogically unsound, and that having several exams — a final plus a midterm or two — would better measure people's knowledge. (It might also help students learn the material better.) I'm not sure that this is so, because I haven't looked into the research; but it seems plausible, and colleagues I trust have said that the research does support this. Let's assume then that this is right. Why then the single-exam format?Well, then, let me propose that most students would prefer no exams at all, with everybody getting a guaranteed "A" or "A+". (Prof. Volokh describes the near-elimination of grades at Yale and Berkeley, and suggests that it is a school's market power which enables it to abolish traditional grading. That's probably true, but is an issue apart from what students would likely prefer.)
Some of the commenters identified one important answer: Professors hate grading exams, and would thus rather grade one exam than two or more. . . .
I think there's a lot of truth to this, but let me suggest an extra factor: I suspect that most students prefer the one-exam structure as well, so that there's little pushback against the professors' one-exam preference, and there would likely be some pushback against professors' attempts to shift to the "better" two- or three-exam format.
Granted, if everybody got the same grade some students would likely resent the students they perceive as having lesser ability getting "A's" just for showing up in class. (Or would you even have to show up? Students probably would also vote down a mandatory attendance policy.) Yet I've never heard students complain that a grading curve is too soft. No small number of law students pick their electives based upon a professor's curve from the prior year.
Speculation that additional exams might increase law student stress is interesting, but the fact is that in most law schools there are professors who hold midterms and I don't recall students wilting from the additional pressure.
Is it presumptuous of me to suggest that graduate school professors should care about which teaching and testing methods are the best means of advancing and measuring student comprehension and ability? That they should implement sound teaching and testing methods even if they prove less than popular with their students?
Professor Volokh speculates that employers don't much care about the accuracy of one grade or another, and presumes that "random noise in the grading probably averages out in considerable measure when you look at the student's entire transcript. Even if a professor views a law degree as nothing more than a rough equivalent to a "union card" which qualifies a student to get employment as a lawyer, I would still hope that the professor would strive to avoid adding "random noise" to a student's transcript.