I can imagine a couple of reasons.

The most straightforward one actually is "capitalism", but maybe if I explain it a bit you'll accept it? :) I expect that the vast majority of the books that these companies are scanning are bought in bulk from wholesalers at the cheapest possible price, since the goal is quantity over quality. That means they're probably buying the leftover books that the wholesalers are just one short step away from sending off to be recycled into pulp anyway. So re-binding them would mean they're still left with nearly-worthless books that nobody else wanted to buy. It's like those book sales libraries have from time to time, they put out the books that are scheduled for "disposal" in hopes that somebody will pick up a few before they go into the dumpster.

The other reason depends on the physical nature of the book. There's lots of different book binding techniques and not all of them will leave you with something that's easy to put back together. Lots of books are made up of smaller subunits of pages called "signatures", typically 8, 16, or 32 pages, that get stitched together into the finished product. There are coil-bound books, comb-bound books, all sorts of things like that. A lot of them probably wouldn't leave pages that are amenable to a one-size-fits-all rebinding. So that makes things a lot more costly, and again that's a cost that produces a book that's probably worthless for resale anyway.

Honestly I suspect that getting chopped and scanned is the best case scenario for most of the books in this situation. It converts them to digital form, rescuing their contents from being lost. The thing that bugs me is just how these companies squirrel away their hoards of books from the public. And the blame for that lies largely on copyright law, so I can't blame them too much.