The reason libraries are thin is that librarians will walk through fire to get you a book when you need it. I've gotten xerox copies of 40-50 pages faxed from libraries in Louisiana to New Mexico back when "fax" was an alien technology. Libraries don't need your books. Think of the stuff on the shelves as the exhibition at a museum, the inter-library loan as the museum's collection and everything else as "someone's old crappy books." We've got a real thing about "NEVER EVER DESTROY BOOKS" but fuckin' hell, "paper" has not equalled "knowledge" for 30 years now and just because the dead tree version is available somewhere does not mean that the huddled masses questing for knowledge will stumble across it on the shelves and be smitten with enlightenment. "Library science" is a graduate degree. My grandmother was a librarian. Wandering around assuming you know more about what should be on shelves than the professionals do for the simple reason that you like books is... naive.
When I was desperately looking for a career ca. 2009 I looked in the public libraries near me. Why the fuck am I gonna spend $50,000 to get a library science degree so I can be qualified to shelve books for $15,000 a year? A master's in library science was seriously a requirement for a $10/hour job. At the time I don't think I'd given up on getting an MFA and becoming a professor which pays fuck all, but I'd be making a thing I could sell on the side. The professional world is a big, dumb, stupid place where a master's in "communications" can net you a VP title at a bank and an equivalent in an actual field can net you Top Ramen for dinner every day"Library science" is a graduate degree.