You might be right that a Sling made out of leather/fur is very different to one made entirely out of strings, but they shouldn’t change that, to be honest. Let me give you some examples:
You can find grenades of all sorts in CDDA, usually from the Military (Military drops, bunker, zombies…) ; Those grenades are high quality, very well made and so on.
The >player< can make their own grenades as well, and those stack with the grenades made from professional manufacturers.
You can find different Melee Weapons, for example Broadswords, Katanas, Zweihänder and so on, all from skilled blacksmiths or professional manufacturers who did this for years. The Player also can smith himself weapons, and it ends up in the same item as the weapons you can find.
You find a lot of clothing ingame, shirts, pants, cloaks, shoes, socks and so on. The player can, with a few rags (usually of different colours) and a sewing kit, make their own clothing, which results in the same item you can find in clothing stores, even though it’s technically made from a dozen rags from different origins.
You can find food like bread, broth, sauces, lunch meat and so on ingame, usually filled to the brim with chemicals to make it last longer, to colour it and so on. You can also make the same food, but without said chemicals.
Do you get the idea yet? The game would need to >double< the amount of items in order to reflect those differences. This would increase the size of the game, it would make the inventory screen a hell of a lot more tedious (“Why do I have 5 different stacks of EMP Grenades?!” :P) And it would probably be slower too because it would need to keep track of tons of different items instead of a more manageable amount of different items.