A lot of these mad old hoarders bought the stuff when it was junk and nobody wanted it...They thought it was treasure even then. Years later people start knocking on their doors asking to buy "See I was right, its really is special, everyone wants it. Worth a fortune so it is!" Their original opinion is reinforced, they cant see that the stuff has deteriorated, just the same as they couldn't see why the stuff was worthless at the time they got it.
Often these old bikes languishing in sheds, but not for sale, are so prized by their owners, because they offer a connection with their past, conjuring up memories of their youth, friendships, wild rides, holidays, early experiences with girls, etc.
To us, they are actually just old motorbikes, to them they are much more.
Cheers,
Iain