Because it is rare. Not only do you need the money, you also need most likely be in a private server in the hopes of not getting killed, then if you DO get a mono sphere from digging (Somehow) oh wait, you need to do an arrow grind for Doppio, unless you get it before going to dig via trading or what. We overprice Oreo because of how hard it is to get, unlike where people overprice GE because it got a rework. It's even a 0.375% chance for a mono sphere. Like I just said, we overprice Oreo because of how hard it is to get.