Sometimes they do have this, but upper management often requires anyone under them to be replaceable lest they have too much leverage in salary negotiation, which undercuts this. Hence it's not an official role, and can easily be broken by the usual biz processes that are insensitive to it. Until a methodology comes along with a specific method of hiring for such a role, this might not change. Corp modelling likes to treat developers as interchangeable commodity units - gatekeeper needs a lot of software-specific context that defies this model.