I was interested in purchasing the Cambridge History of Greek and Roman Warfare, which seemed like a great two-volume history of ancient warfare, but the two books are $200 and $160. And the used versions aren't much cheaper. You're never going to find books published by Penguin or Basic Books that are nearly this expensive. Why are academic books priced like this? Who's supposed to buy a $400 history of ancient warfare?
The answer to "who's supposed to buy" those at those prices is "libraries and not individuals." They do not expect to sell the books much beyond libraries, but suspect that libraries will want them, and so charge ridiculously high rates in an attempt to recoup the costs of producing, printing, and storing the book, as well as staff, as well as losses on other academic books that don't even sell enough (at high prices) to make their money back.
Reference books in particular (like the one you are mentioning) are particularly prone to this, because they really believe that their ideal audience is someone already at a university who will lobby their library to buy a copy. Does this decrease the spread of knowledge? It does. Is that deeply antithetical to the mission of academia? It is. Is that how the system currently works, even for non-for-profit publishers? It is. Are there ways around it? You can find a library that has them, or you can look in less-than-legal places to find copies. This is what many (most?) academics do, even those who have legal access to these things (but find the legal means cumbersome to use).