This have to do with how the treasure map works. In reality, treasure map in each zone is actually a different object. This is proven via black market, when it is available to buy. You will see that the amount you own is a tiny number even if you have a lot. Giving it directly would require randomization on the giving part so it will require a longer reward list. But giving it in a pack, they can offload the random to the pack and make it easier to handle.