Because the vast majority of people don’t have a reason to do it. They’ve never used Linux before - heck there are people who have never heard of it before.
The other thing is you and I, chances are can find a use for our old machines, have a place to store it, or know how valuable it currently is. Most other people aren’t aware of how parts or entire systems depreciates, don’t have a use for a second computer, and can’t afford the storage space to store a spare PC for a backup. They also don’t really have time to do a lot of research on the issue or just plain old don’t care.
So what do they do? Well there only remaining option is to throw it away, maybe theyll be a bit wise and take it to an electronics recycler, where you have to trust it won’t get thrown away anyway.