Who says RPG manuals don't count as books?
I suspect lots of people, if not the majority, buy the books, skim them, and never play them. We just don't like to admit it.
Some of the books are very long, and lots of D&D fans are busy (full time jobs plus kids, girlfriend plus school, etc) I barely see my friends now, can barely keep up with work-related reading, let alone play RPGs. However, RPG books get more beautiful and cool every year, there isn't really anything like them. I am curious to see how the game evolves across editions. The new classes, new monsters, new adventures. I just like them, and I now have the money to buy multiple $40-$50 books at a time.
I, too, stopped playing at 2nd and now just collect the books. I get D&D, Star Wars RPG, Numenera, The One Ring, etc. I will never play them (maybe when I retire), but the books are works of art. I love Star Wars and LotR, and these books shred information on them in addition to the novels and movies. I just love collecting them and supporting the hobby, but I am mad busy running my family and business. Nothing wrong with that.