I've wondered about this too. Did Ridley Scott and crew set out to make the greatest movie ever (IMO)? Or did something magical happen? My guess is "auteurs" often set out to create a masterwork but usually something beyond their control derails that. But, if they are lucky, they stumble upon something extraordinary that touches the heart of the human experience and that only happens once in a lifetime, if even that.
As an example on the other side of the spectrum, there may be plenty of bad and forgettable movies with incredible and unforgettable performances from a very small part of it (the sound recording or design, one of the actors, a single superb scene, one of the scenerys) while the rest is mediocre to bad.