I think part of OPs point is that you think you can get a copy of any book. I don't mean to sound snarky at all; it's just that you know what you see online now, which is not all that is offline. You don't miss what you don't know.

My experience is of numerous out of print books that might have had small publishers in small editions. Even an unusual edition of an well known book can be unique in various valuable ways.

One year the university I was at was in danger of flooding. The priority was moving books that as far as they could determine were the only remaining copies of. You wouldn't believe the numbers of them that were moved. Many used bookstores I've visited were similar, with books selected because they were unusual.

It's hard to know what you don't know.