I’m going to try to explain a mystery that puzzled me for years. This answer finally dawned on me in the middle of one of those occasional conversations in which non-perplexed friends patiently try to explain the issue to me. So I am not sure if mine is a novel explanation, or merely the explanation that my friends were trying to tell me, in which case my contribution is explaining it in a way that is at all comprehensible to a person like me. If it is novel, apparently some other people disagree with it and have an almost entirely satisfactory alternative, which has the one downside that it is impossible to explain to me.
The puzzle is this:
Why do people read old philosophers to learn about philosophy?
We read old physicists if we want to do original research on the history of physics. Or maybe if we are studying an aspect of physics so obscure that nobody has covered it in hundreds of years. If we want to learn physics we read a physics textbook. As far as I know, the story is similar in math, chemistry, engineering, economics, and business (though maybe some other subjects that I know less about are more like philosophy).
Yet go to philosophy grad school, and you will read original papers and books by historical philosophers. Research projects explore in great detail what it is that Aristotle actually said, thought, and meant. Scholars will learn the languages that the relevant texts were written in, because none of the translations can do the texts the necessary justice. The courses and books will be named after people like ‘Hume’ as often as they are named after topics of inquiry like ‘Causality’ and larger subject areas will be organized by the spatiotemporal location of the philosopher, rather than by the subject matter: Ancient Philosophy, Early Modern Philosophy, Chinese Philosophy, Continental Philosophy.