Because crafting makes any future drops of the same weapon completely and utterly useless once you have your 5 red borders. This way would put emphasis back on the excitement of a new drop that had a perk you want as opposed to just simply an automatic dismantle... People just seem like they want everything handed to them now and then just complain when weapon drops don't seem to have any meaning (Not saying this applies to you but just people in general these days).... This way, you would still ultimately get your desired rolls (even more on one weapon than we currently are allowed now), and would always allow for that salvageable weapon to be worth something.