Did you watch any pre-Star Wars 70's sci fi movies? They aren't exactly full of unicorns and roses.
Quite so. The '70s were a very dystopian time for sci-fi movies. Star Wars totally reset the board and redefined the "default" SF/fantasy/space opera movie as a lightweight, kid-friendly fantasy adventure with lots of action and visual effects. There were many critics and fans at the time who despised Lucas's and Spielberg's films for their dumbing-down influence on screen SF. Although in retrospect, they deserve credit for bringing back some fun and optimism, even if it's the kind of superficial optimism based on the good guys winning by killing as many bad guys as possible.