It's culture. These scans are of course not distributed, so the value doesn't quite continue to exist.

You say "culture", but is is culture, and culture that was interesting enough to write down. Writing a book in the 70s isn't a trivial thing, there are gatekeepers, editors, publishers etc., even for your little book about local history, and the information is valuable to the community it's about. It affects their internal cultural transmission.

The physical work before was never distributed either (which is what makes them economically valuable), so the the point is mostly moot. I say mostly because at least when these neglected works become part of a training corpus, the knowledge they contain can be surfaced on demand, or even by accident. Think of it like the grandpa telling stories of his experiences to the grandkids, to the best of his recollection (and imagination), going on related tangents as they occur to him or are triggered by the grandkids' questions, etc.