The Anatomy of Story: 22 Steps to Becoming a Master Storyteller by John Truby is arguably the first book a screenwriter is supposed to study to learn screenwriting. So it is safe to say that the guy knows about screenwriting. And after years of world cinema experience, It is also safe to say that any movie with a strong screenplay can never go wrong, no matter how average the cinematography is or how dark the tone is or how bad the CGI is.
Considering all this, it is very surprising to know that John Truby does not have a single movie writing credit to his name (IMDb - John Truby). In this day and age when studios are willing to bet millions on single project and getting them reviewed by advertising agencies for crying out loud, it makes sense for them to have their script reviewed by someone who has years of experience learning hit and flop movies and who can look at the script objectively and point out the flaws to the creators.
Why doesn't this happen?
Edit : I would like to clarify that I am really looking for specific instance/s where this might have been tried by someone in the past which didn't go well or any other thing might have prevented this from happening because of the way movie-making business works which an insider might be able to point out.