They're valuable because they're rare, because famous people use them, and they're extremely well hyped.
Also, from what I've told they're very thin and stiff. Which is what makes them so popular with people that have enough money to afford them.
However, they're only valuable to the (comparatively) tiny magic and cardistry communities. It would be SOOO not worth it to the USPCC to put together the old presses with the old methods so they could print cards for a tiny group of people (in their eyes). It seems like everyone would pay so much money for them because on these forums we're surrounded by people who value them so highly, but generally, people think it's INSANE to pay that much for 54 pieces of paper.