Probably because being able to craft two rings, a belt, and a necklace would all lead to severe min/maxing. From what I've observed from things devs of various games have said is that they prefer that their players not get too carried away with min/maxing and that they don't like a particular class and build to be far and above THE BEST way to play, because that ultimately reduces options if players perceive that they can only "win" by playing a certain way. This is also one of the reasons for continual balance changes throughout a game's lifetime.
Now, you may not think that this is something the devs should consider, particularly as an aspect of a SP game, but the fact is that it is something that they do consider.