They would have to make several thousand different models; combinatorial mathematics tells us that this would require almost infinite work.
Making a game is hard. They should pay people to make them for a living, oh wait.
We don't know how many items there are, like I said, you can't definitively say how much work it would require. An "infinite" amount of work is such an exaggeration. Simply putting a texture on the weapon would look better imo. It doesn't HAVE to be different models.