It's easier to get good quality scans from individual pages than it is from pages in a complete book. Imagine laying a book flat, then the page is distorted in the area around the spine. You can work around this (either by trying to correct for the distortion in software or with clever scanners that position the book more advantageously) but it adds complexity compared to chopping off the spine and just dealing with flat sheets of paper.
Easier to send them through a duplex scanner (or put them in a flatbed one if they are fragile). Cheaper than buying the automated ones with the page turning robot arm.
I have done it at home for my books since the mid-00s.
Not OP, but I do similar. I only de-spine a book if it is in poor condition (and I have a nice copy). Some of the material I scan is stapled instead of glued and I can remove the staples, scan and replace with new (not rusted!) staples to return the book to its original form.
I have books but would have 5x as many if I could not capture them digitally. (When I go to move or go through a purge, some of the books I scanned do go to a used book store.)
So I scan in part to keep my physical book-footprint smaller, but also my scans all get cleaned up and uploaded to archive.org. Mainly I scan young-adult science books from the 50's and 60's (since they were so influential and have all but disappeared except on eBay and the like).
Not OP, but: If it's something I want to work from, I get a printer to slice the spine off and rebind it with a ring binding. That way it lays flat on the desk.
You've trashed the book (and its lifespan) but some books are for using, not keeping.
Sometimes? But for scanning I can use a cheap worn copy as long as the pages are clean.
Most books I would rather have a digital copy. I had four or five book cases full of stuff and it makes moving difficult.
I originally started scanning 20 years ago when it was more difficult to find PDFs of basically whatever you want. Now it is only random old books I find at the used shop or online for cheap and somehow cannot get from archive.org or elsewhere.