Pretty sure it's just because if they made them tradeable, then they could be used by RMTs. Even if they restricted card-trading to a special window that didn't allow gil.
An MGP-based Market Board would be interesting. But I don't know; I don't know how much trouble an RMT and their buyers would be willing to go through to find a roundabout way to transfer MGP through it.
I think it's fine if they do leave it this way, but the rates would need to be upped.
