No, it's not typical to scan books before throwing them out - unless the book is important
I've seen a number of answers saying how unfortunate it is that libraries don't scan them, and this strongly underestimates the work involved in a useful scanning project.
I've spent roughly three years working as a developer on a large digitization project for an institution with roughly 300 years of important archives. Getting a useful scan involved these steps:
1) Working out copyright/existing agreements:
This isn't an easy step. Some books can be in the public domain. Others need the copyright holder to be contacted, which may take some considerable detective work, in the case of older works.
2) Preparation of the source material:
This is typically done by an archivist, possibly working with a bookbinder. Typically, in the past, you needed to remove the book spine, to turn it into flat sheets - it's a bit less necessary now as there's software that will work out how the page bows and flatten it, but for good quality scans removing the book's spine and restoring it is still the gold standard
3) Scanning: To do this at any sort of speed takes a large, highly automated, and expensive setup. Our test system could do 1 page every minute. We were up to 10 by the end of my involvement in the project, and this required 2 full time employees working with the material (admittedly, these were unique items, you could go faster on books going to be thrown out anyway, but you still need to ensure decent image quality
4) Doing stuff with the scans: This is where I come in! having a giant server full of book images accessible to the public is fine, but not very useful. We'd typically add metadata to each book scanned, we might do OCR on the documents, which is pretty imperfect still, so needs someone to skim through for mistakes. If any of the books are being released under a licence, you need to put access controls in place.
5) Keeping funding for the project so all your hard work doesn't just vanish: surprisingly, the hardest step out of the 5. Academic institutes are normally happy to commit funding for long enough to get a nice news headline, and have a thing to show off at the next benefactors meeting.
Basically, the TL:DR of this is that doing this kind of project, even on a small scale, takes a lot of person hours. Your academic library probably has limited resources to preserve, say, older textbooks - they might reasonably argue that if the material is still covered in modern textbooks, and the book isn't of particular importance, something else should get priority.