The creators said they didn't let her be an optional player because she's powerful enough to take out the survivors one by one or something like that. If they did allow her to be chosen, a lot of people would choose her just to slay someone with one strike lol
If you were to play the Witch it would be far too short. There would be too much sitting around, and choosing where to spawn would be difficult. Once you'd get startled, it would be short and there isn't too much you can really do. There are a lot of different reasons. She's just not the type to be played as.