I'm just stating that not everyone is reading the most popular million books. And, there are many, many millions of books in the long tail.
A few years ago, after running into this issue several times, I looked into the economics of it to see if there was a opportunity to republish digitally. I found the sales data for some, the ones that had value were selling on average for around $50-150 dollars. Because the typefaces weren't modern, OCR wasn't scalable. Because only 20-40 were transacted each year, it wasn't worth the time.
What kind of books are they? In my case mostly historical documents by some relatively unimportant person who was highly important for a very, very niche subject. They still contain valuable, irreplacable information.
An analogy: imagine you discover a really cool video game from 20 years ago. You love it and want to find the developer's previous work. But, you find out that the company was purchase by another company which was purchase by another, etc, etc. Sure, maybe the game still exists in some digital vault. But, more than likely it's gone because old things only seem have value these days if some influencer broadcasts it.
It's interesting that libraries are purging them. Several times, I've found that the only available copies were at some random university rare book collection in middle of no where, and basically impossible to access unless one fly in - which isn't worth it. They are often donated by a benefactor and stuck there - which is why they're never read. It's not that the content isn't valuable.