Why are old chess books worth less money?

I noticed on online websites that opening books published in the 1970s or 1980s are not sold at very high prices (maybe a couple dollars).  But opening books published this year or last year might be between 15 to 30 dollars. 

I don't understand why older books are selling for so less.  The chess game has not changed.  For example, people were playing the Sicilian or French defenses back in the 1970s and I am sure those books written back then provide the same details as the ones today.

I can understand if medical textbooks or law school books from the 1970s are not worth anything because medical advances or new laws or law theories would make the material obsolete.  But chess is a boardgame and I cannot imagine that the boardgame has changed so drastically.

They're certainly useful, although be prepared for descriptive notation. I picked up an old Fred Renfield paperback at Half-Price Books for $5 last weekend and it was worth every penny.

I literally paid with pennies.