The definition of sci-fi is science FICTION. This doesn't necessarily mean high technology, it means ficticious technology. Technology that doesn't exist. The first noteworthy piece of sci-fi litterature is, believe it or not, Mary Shelley's Frankenstein. The book dealt with the re-animation of corpses through electricity, as it had just been discovered that if you stick a metal pin in a dead frog and then send electricity through it, the corpse will begin moving spastically.

Sci-Fi based games similarly don't have to be high-tech. Take Parasite Eve for example. The game revolves around a "what if" based on a widely known scientific fact, that Mitochondria are a symbiotic life form which exist within every cell in the human body. Take a scientific fact, put a ficticious spin on it, and boom. Instant sci-fi. The fact is, we already HAVE sci-fi RPGs, and tons of them. I could probably dig up tons and tons more if I felt like it.

Space operas? Yeah, it's true, there aren't too many of them in RPG from. They are out there though.