Star Wars was a product. Think of all the Star Wars that never got made because no studio would pick them up. Think about the vastness of human experience and commentary that exits outside of white guys in Hollywood, CA in the 1970s/80s. You're thinking too narrow. There could have been much more and overt anti-imperialist films being made and Star Wars would have been one thing in hundreds or thousands. The fact that it was a brand meant it had to be reinforced in culture and continued to keep churning profits. Thus it was further embedded in the mythology of Hollywood. Plus they were fine with making an anti-imperialist movie because Americans never saw themselves as the Empire. It was modeled after Nazis. Anti-imperialism was fine a long time ago in the US because it specifically meant anti-European imperial powers. The US wanted to cement US capitalism as the world order. JFK was all about anti-imperialism. Though we all know that American capitalism is just another form of imperialism, liberals don't. The average person doesn't.

Star Wars is not anti-imperialist because the movie had rebels fighting an evil empire. It's a product of empire. You can't separate it being a product of empire from the story or plot or message or whatever else. The idea that Star Wars planted a seed of anti-imperialism and therefore that's why people are leftists now is pretty immaterial. Don't get lost in idealism. People were anti-imperial before Star Wars but for different reasons than you think (as I said above) and people were anti-war because they could be drafted. Then, after Star Wars came out, Americans very much became okay with being the Empire. In fact they doubled down on it right around the time the Prequel Trilogy came out. Revenge of the Sith came out in 2005, during the height of Americans wanting to crush the rebels for blowing up their towers.

Movies are downstream from politics, not the other way around.