[Butz]

Think of it this way:
A warrior comes into possession of a powerful artifact, carries it with him all the time, and then leaves the lands for somewhere else. A duplicate artifact isn't just going to appear out of thin air because the warrior took it with him. That sort of defeats the whole purpose of limiting items in the first place.
And you can never really be sure that the person who left with the item won't come back some day. Accounts can always be upgraded.