First, $60 is not really that much for such a book. These days, $20 (which you mention) is the price for a relatively small paperback book. For example, my most recent purchase is Reality is not what it Seems by Carlo Rovelli. This is a short book (c. 250 relatively pages) and it's in paperback and the price was $18 in a regular bookstore. Another is The Investigator by John Sandford. That one is a mystery and truly mass market. It's a very small paperback and about 400 pages. Price was $11.00.
Second, the economics of the book business are such that the author will get a relatively small portion of the cover price. I don't know the exact numbers (which will vary by book, author, and publisher) but I would guess that an author is lucky to get 20% of the cover price and 10% might be more likely. So, that's (say) $10 per copy of that book. And books in niche areas are unlikely to sell many copies. And they take many, many hours to write.
Third, others have mentioned some of the audiences for the book. But, for sure, they are not the "average reader". But I'd like to make another point. I also like to read philosophy. If you look at "price per book" they are indeed expensive. But if we look at "price per hour" they are not. The Sandford book I mentioned took me about 6 hours to read. That's $2.00 per hour. I haven't read the Rovelli yet, but I'm guessing it will take me about 5 hours, that's $4.00 per hour. An academic philosophy book? That will take many hours. They are typically quite long and often fairly dense. 20 hours would be $3.00 hour, and some such books take me longer than that!