One thing to keep in mind is that many (most?) of the books and papers in these archives are decades old, usually no longer in print, make zero or vanishingly small amounts of money for their original creators, are sometimes only physically available from distant libraries that are challenging to access, etc.

In doing scholarly research, it's extremely helpful to be able to quickly search and skim hundreds of vaguely relevant sources, but simply wouldn't be worth the trouble to pay for or track down a "legitimate" copy of every one, and in many cases would be physically impossible. These "pirate" archives make doing real library research, previously limited to scholars at top-tier universities, accessible to orders of magnitude more people.

There really isn't that much profit in most of these works, and whether a scholar reads one on their laptop screen vs. in a physical book in a university library somewhere doesn't have any material impact on the original authors, editor, illustrator, translator, printer, etc.