A lot of the buyers in any collectable market are just people who think scarcity and value are the same thing. They tend to buy scarce stuff to hopefully have it accrue value over time.

The problem in this case is that two unreleased games are only scarce specifically, not generally. It would be one thing if one of these was a 90% finished prototype of a Metroid sequel that never existed, but there are a lot of unreleased games out there that were never released because people do not want them.