I think ordinary people keep owners manuals, but don't bother digging for them when they sell/get rid of the item.
When I was doing my grandpa's estate sale, he had all the owners manuals in a filing cabinet, so I was able to pair the manuals with the items. A guy offered $50 for an old craftsman miter saw, and I said, "Look buddy, it's old, but it works, it has ALL the accessories, and it even has the owners manual! It's worth at least $60."
I got the $60.