Speaking as an author: because I write for a living, and if I can't earn a living at this I'll have to find some other job, which will cause me to stop writing. Piracy is directly rivalrous for the author's revenue-earning product. (Libraries are less so insofar as Public Lending Right and royalties on library editions cause some money to go to the authors.)
Remember, authors are not film studios or rock groups. We don't have merchandising opportunities, we don't perform live for a ticket fee, and we're mostly very small-scale sole trader businesses. Our revenue comes 100% from sales of our books, unlike bands (who make peanuts from streaming but are paid to tour) or film studios (lots of spin-off merchandise).
(Median income for full time novelists working in the UK, per the ALCS in 2022, was around £7000 a year. A tiny handful make millions, a large number are hobbyists doing it in their spare time for pin money, a small number of us earn a middle class income -- not management level, not even programmer or web developer level. It doesn't take much to tip us from "just getting by" to "giving up, have to go stack shelves in a supermarket" territory.)