I think it's just tradition - always been that way and string makers don't wish to change in case it puts purchasers off. Originally, stringed instruments would have had wooden pegs, so the silk helped stop them biting into the pegs. Even with metal, the winding doesn't usually extend to the part that wraps around the peg, so the thinner core under tension would be more likely to mark/damage the peg, whether wood or metal.