Talking about recent works, released in the past few years getting pirated specifically:

First is the unfairness of the system. Disney and other publishers have lobbied to push copyright up to the ridiculous life + 70 years using their political power. Publishers collectively have a totally ridiculous entitlement to governments being compelled to spend public money on enforcing their monopolies. So if you're say living in poorsville, you have no realistic opportunity to wait for your book to be published for free, because any work that is copyrighted during somebodies lifetime will be copyrighted until they're dead. Sure there's libraries but if your book is obscure enough it may not be available in one.

The second reason I'll give is accessibility. If your book isn't offered in a free electronic format from a library, and somebody is blind, and they can't afford the kindle, they're out of luck. The publishing industry has no problem being exclusionary towards disabled groups so long as it enables a better profit model, pirates on the other hand are totally inclusive and scan everything making everything OCRable.

So besides accessibility/fairness arguments, I'll also bring up that paying for books is a really shitty way to support authors. I paid $250 for physical books this week. Am I a hero to authors? Not at all. Maybe $200 of that went to the estate of some authors and most went to the publishers. $50 of it went to authors maybe closer to scraping by, for whom piracy can mean not putting food on the table, it means having to give up authorship. However of that $50 maybe $5 went to such authors. So I paid $250 and gave $5 to authors who really needed it. Does this prove I give a shit about authors? I think it proves that I don't, if somebody cares about authors they'll pirate the ebook and directly donate $250 to them.

Finally there's the issue of how books are secured. Frankly the legal methods for acquiring books are frequently broken and bad. DRM that auto-deletes books from people's devices is an abomination. Waitlists for ebooks are the stupidest thing I have ever seen in my life. A huge reason people use zlibrary is their local library has broken software and it's like 8 steps to download something. Most of the money is in Amazon, so that's what gets funding to make the ebook/audiobook experience as seamless as possible for the wealthy of the world.

My proposal to fix all these issues would be for public libraries to require public ID authentication to download books, for copyrights to essentially ignore copyright, and then based on which books are being downloaded to compensate authors out of a public fund. This would not come with any enforcement efforts to shut down pirate sites (due to censorship/privacy concerns and concerns with the library's software being broken, not everybody has to use the library just a good number of people do), the libraries would essentially act as a voting system to direct monies to different authors which you could also use as an e-book distribution mechanism. This makes the availability of books for the wealthy contingent on them financing books for the poor, the disabled, and compensating authors.

Until such a day happens, if somebody pirate $100 of a books and donate $30 to some random author I think they're a better person that somebody who buys legally. We're in times where lawlessness enables the most ethical option available, so you know, maybe the law is screwed up.