Even if its not pirated, from popular RPGs you will find the stuff online for free somewhere, either in a wiki or some other form. (Digital tool etc.).Piracy has always been an excuse. The books are pirated anyway. OTOH, a PDF, once bought, is in the hands of the buyer. DDB stuff lives in their enclosed and proprietary format and it can be changed and removed at any time.
Sometimes people even first look at PDFs and then buy them (of course this happens less when they suck).
Honestly I would never buy only a book from an RPG ever (and I hope books die soon anyway to allow PDFs to be better and not just book copies...), but I do buy PDFs. And I dont think that the book enjoyer people suddenly stop buying books, because there are PDFs around.
Today even a lot of comics are made for digital formats first (in scrollfriendly formats), and people still buy physical books of them afterwards.

