Not copyright, as much as if the book isn't precious, it's easier to do that, feed the loose pages into the scanner, and then get an intact one if you want it, compared to the additional expense of having to build and program a machine to carefully turn the pages and photograph what's inside, or the time it would need by comparison.

It just doesn't work if the spine is still there.

I can't imagine that scanning 'every book in the world' would require filtering, unless a ham sandwich or Nintendo 64 game has a chance of jumping into their production line then 'If book, then scan' is the only filter they need.