For most of them it’s not “laziness” but ignorance. They don’t know it’s an option, they don’t know why you’d want to do it, and the don’t know how to do it.

In some interpretation of copyright law, it’s considered a “transfer” rather than a “copy” if you destroy the physical original. (Ignore all subsequent copying of the digital version.)

Actually old and rare books are probably public domain, so that’s not an argument.

Also, most of the books they scan/destroy are low quality trashy novels that won’t even sell at thrift stores. But as they are written by humans it is still useful for training models.