SDK and modders just offer higher quality stuff. Can you imagine game developers releasing something like the initial Counterstrike, Natural Selection, Team Fortress for free ? No chance in hell. If they knew the stuff was going to be popular, they would try to sell it as another game. Classic example was Wolfenstein: Enemy Territory. It was meant to be commercial and only was released for free because it was canceled. The best they (paid developers) are going to offer you are beautifully modeled, animated, and skinned weapons/characters… that are bland and don’t make the game more interesting.
All the wildly popular modifications and additions to games are creations of modders, not paid developers. Show me a counterexample.