Da sua pergunta, a parte principal é:
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?
Sim. O tempo todo. Se você olhar a biografia de Truby disponível em seu site :
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.
(ênfase minha)
Para alguma explicação, você precisa ver quem realmente escreve créditos para um roteiro, e alguns dos regulamentos envolvidos (devido a contracheques, créditos e resíduos). De um post no 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.
Então, enquanto ele olha para roteiros de filmes e faz críticas, sugere reescreve, o que quer que seja, ele não está mudando o roteiro o suficiente para se qualificar para um crédito de tela.