Because having a non-vassal Savoy sitting there with just 4 provinces (or more from selling lands to him, but why would you do that if you have to reconquer him later?) sounds very counter intuitive to get as many Italian provinces as you can in one day.
But vassalizing him will cause the mission to pop up, so you're better off just killing him then sell those lands to someone who doesn't trigger the annoying annex missions.
But vassalizing him will cause the mission to pop up, so you're better off just killing him then sell those lands to someone who doesn't trigger the annoying annex missions.