VooDoo Innovations sells a BC with an integral gas key, machined as part of the overall carrier. Wonder why more folks aren't doing this? I own a couple of them and like them very much.
If I had to guess I would say that the cost to manufacture is high and you lose the ability to replace failures in either part while leaving the other. If my gas key fails, I can. Just bolt on a new one without having to replace the entire bcg.