I think sometimes biopics are made so that actors can play famous dead people.
But the question is how can a movie about a great/famous/legendary filmmaker do anything but somehow, in a tiny way, diminish their films because a film about Hitchcock, not by Hitchcock, not only be of lesser quality and depth than any of the films he made, but also not a Hitchcock film?
And aren’t Hitchcock’s films about himself anyway? Figuratively and literally he put himself in (almost) all of his movies. On thematic levels Rear Window is about his anxiety about the voyeurism inherent in making the types of films he made, and couldn’t Vertigo be read as being about his relationship with actors, his love for Grace Kelly, and punishing Kim Novak for not being Grace Kelly.