Why doesn't John Truby write screenplays? [closed]

From your question, the core piece is:

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?

It does. All the time. If you look at the biography for Truby available on his website:

Called “the best script doctor in the movie industry,” Truby serves as a story consultant for major studios and production companies worldwide, and has been a script doctor on more than 1,800 movies, sitcoms and television dramas for the likes of Disney, Universal, Sony Pictures, FOX, HBO, Alliance Atlantis, Paramount, BBC, MTV and more.

(Emphasis mine)

For some explanation, you need to look at who actually gets writing credit for a screenplay, and some of the regulations involved (Due to paychecks, credits and residuals). From a post on Slashfilm:

And to get credit, a writer must contribute a certain percentage of the finished work:

Any writer whose work represents a contribution of more than 33% of a screenplay shall be entitled to screenplay credit, except where the screenplay is an original screenplay. In the case of an original screenplay, any subsequent writer or writing team must contribute 50% to the final screenplay.

So while he looks at film scripts and gives critiques, suggests rewrites, whatever, he is not changing the script enough to qualify for a screen credit.