Many of these publishers have standing orders with university libraries where the books are bought sight-unseen. This is particularly the case with some of the major European academic publishers like Brill, De Gruyter, etc (at least in the humanities). So that means that university libraries have these books shipped to them the moment they're available for sale.

For example, my university reference librarian for my field (ancient history/biblical studies) has a standing order to purchase every book in about 5-6 different series each that Brill, De Gruyter, Eisenbrauns, and more put out. They even have standing orders for multiple books in the link you provided (Sinica Leidensia, the Handbook of Oriental Studies series).

Because they are able to so easily sell these books to libraries regardless of cost, they don't feel the need to provide it at a price-point for the average person/academic. When you can sell it to 1,500+ university libraries for $200, they don't need to sell it for less. A lot fewer people who are not associated with a university are willing to purchase (or even read) these books than you would think.

My personal advice would be to abstain from buying book like this. Buy reference works; don't spend the money on stodgy, overpriced Brill books. You'd be shocked at how poorly made, edited, and varying in quality many of these works are.

u/Georgy_K_Zhukov also answered a similar question in yesterday's Short Answers to Simple Questions

The question is already well answered, but as a piece of general advice: if you see one of these books that you really want to read try reaching out to the author (via Twitter or institutional email if they're a full time academic) and see if they'll share a near final proof of the book in PDF with you. They generally don't get royalties for these books and many historians are more than happy to share their research with and interested reader. Not everyone will of course, but you'd be surprised how many will!