I would assume they aren’t bothering to scan it if it’s in the public domain. Much easier to just grab a copy off archive.org or any other website hosting such things, and that’s assuming the entirety of the public domain isn’t already in the training set.

IIRC destruction of the physical copy to prevent resale while still possessing a copy is required for book scanning to be free use, or otherwise not encounter problems with copyright law. Proceeding to share/distribute those scans, or especially trying to charge money for them, would encounter those exact same copyright barriers.

While that would be nice, unfortunately it would almost certainly run into the very same legal issues that are also causing them to pulp the books after scanning them.

And you’d be safer for it.