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.

Speed and cost. You can run the book through a typical sheet-fed scanner instead of using a contraption like this: https://linearbookscanner.org/

As for consumer-grade solutions, look for the Fujitsu SV600.

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.