If you're going to just borrow it from a library, then why not just pirate it? The author/publisher isn't getting paid in either case, and libraries are 1000x less convenient than pirating.
The authors and publishers are getting paid by the library for the physical book borrowed, which endures wear and tear and must ultimately be replaced. Not sure how licensing for digital books work with libraries - all the library systems I've used have a cap on the number of digital books that can be lent out.
