What is the metric for "greatest sci-fi directors of all time"? It sounds like they're just comparing the most successful old scifi movies. I have a soft spot for some older sci-fi movies, but most old sci-fi film is incredibly corny and not worth wasting time on.
The best sci-fi doesn't come in a single hour and half long film - it comes from season long story archs with several integrating sub plots.
I know there are a lot of gen-X and older out there that like to argue for the self-contained world of episodic Trek, but that format needs to die. It's boring.