Thinking back on the Golden Age authors I can only think of one that was liberal, Isaac Asimov. A better question is why are most of all movies liberal? Scifi is probably less liberal than most movie genres.
Avatar was a sort of joke. A $500 million dollar joke. I’ve never seen a more visually impressive while abysmally stupid movie. “Plan 9 From Outer Space” was a monument of creative ideas compared to Cameron’s “Dances With A blacklight”.
Nearly all the “Golden Age” authors were either libs or socialists. Read Heinlein’s early works. He was an unapolegitic FDR backer. I think Sturgeon was a communist party member. Asimov stayed a liberal while most of the rest changed.