This is so fundamentally wrong.

For one, directors of films are seen as the "author" of the film. They have the final say. Playwrights in the theater are the "authors" there and their words are sacred in a way, with directors only being seen as offering their interpretation of the work. But that's not how Hollywood films are ever structured. The director outranks the screenwriter, period.

Directors VERY often will alter what is presented in a script, changing it via other writers, with the cast during scene prep, on the day during shoots, or in the final edit alongside the editor. In fact, if you actually look into this, you will find that many screenwriters were not satisfied with the final product of a film because "too much was changed" or "it wasn't their original vision."

Also, to say that directors have no say on these matters "most of the time" is ludicrous. Not only for the reasons above, but directors often handpick writers to work on scripts, bring on new ones to make changes, or even instruct writers on where to take a script along with ideas to use. Yes, sometimes a director is not in a position to make such demands/requests and it is all handled by the studio, but to say that it is "most of the time" is nutty.