Even if the models are separate, that might not make it a simple thing to change it. They may still have to go into every model and add instructions to make the "off-hand" section vanish if a separate item is equipped there. And they would have to create a new item for each piece of off-hand gear.

And as wereotter said, that might well be judged as not a worthwhile thing to spend their time on - and possibly an unnecessary increase to the overall file size of the game, if it has to store these "new" item models separately to their identical originals.