Because the main source of ire of book authors is that they lose some part of the revenue from book sales due to copyright infringement, which many call piracy.
Now, piracy is an incorrect and am emotionally charged term to describe this phenomenon. I believe that a productive discussion on any matter requires clarity of chosen terms. Piracy is not one such term when talking about something not related to sea and ships.
What difference does it make if it is copyright infringement or 'piracy'?
We're talking about incentives for authors and how they can fund their risk/time for publishing a book.