Power of books, man. I think subconsciously people understand that the digital written word is somehow susceptible to tampering and deletion.
Plus, books are personal items, far more than so than DVDS or CDs/Records/8-Tracks* etc. I just don't want to go back and read Stephen King's "Christine". I want to read *my* copy of "Christine".
(*I do accept that music moreso than movies have that personal attachment, but the album covers and media itself is still far less important than the actual music.)