[Quoth]

Not to run libraries. Not unless they are mad. I've written both Video rental library and book library software. It's quite different in what it needs to do compared with Calibre.
You may have multiple copies of the same book and need to track them separately.
Lending ebooks is more complicated.

No doubt some Librarians use Calibre for their own ebook collections. An ebook collection manager like Calibre isn't the same kind of thing as actual Lending Library software, also an actual Lending library (physical and/or ebooks) isn't the same as a personal collection.