Science Fiction is about the future.
The reader might object to this: Science Fiction can be about the present, can be about the past, or can be about imaginary worlds. That is true, but for now, I want to posit that Science Fiction, as a genre, is about "The Future". It is about how changes in science and technology will change the future. And to do this, it takes a trajectory of something that already exists, and asks how it might change society as it develops.
That is the first paragraph of any encyclopedia article on science fiction. I want to talk a bit more about that trajectory, and how it deals with time scales. First, we have near future science fiction, epitomized by authors like Philip K Dick and the cyberpunk genre. We have intermediate term science fiction, which is basically Star Trek. Then we have far future science fiction, with the most well known example being Dune, although the lesser known Dying Earth novels of Jack Vance would be another example.
I think that in the general imagination, the most typical form of the genre is the intermediate term science fiction story, the one that takes place a few hundred years in the future, with human beings living in a society and government that is a natural outgrowth of our present time. As I said, Star Trek is the perfect example: the idea of enlightenment science and social values, exemplified by a thinly-fictionalized United States, continuing the Age of Discovery in space. The trajectory between the founding of the United States of America(between the 1600s and 1700s), its development as world power (around the time of the series making, in the 1960s), and its position as an intergalactic power (in the 2260s) has a pretty symmetric time frame. Other examples could be found.
Of the far future science fiction that I know of, most of it doesn't seem to be based on a "trajectory". Take, for example, Dune, set in a future so distant that the exact year isn't important. Dune features scattered references to religious or social concepts we know of ("The Orange Catholic Bible", "Zensunni Wanderers"), but so much time has passed that all of this past is just background material. "Dune" could be an alternative universe as much as it could be the future. The same is true for the setting of Firefly, where humanity has, in some sort of crisis, left earth, and found an extended star system to settle in. The politics and society are in someway parallel to our world, but there is no direct technological developments or historical events referenced: it is basically a reimaginining of human society as a background for what is very literally a space western.
Human society has been around a long time, although this fact isn't always recognized. Ancient civilizations, four or five thousand years ago, were capable of creating monumental architecture, dealing with distant cultures, and thinking about their place in the cosmos. It is easy to view those societies as archaic, as basically unrelated to our modern world, as merely providing some epic scenery as a backdrop to modern scientific progress. But there is a direct line between the societies of Egypt, Mesopotamia and Harappa, and our modern technology.
For example, one of the problems in science-fiction is contemplating the time scale needed to reach our closest stars using our current, or foreseeable technology. To reach Alpha Centauri would take a probe like Voyager around 20,000 years. This seems like a prohibitively, even incomprehensibly, long time. But in another perspective, this is four times as long as since the building of the Pyramids, or twice as long as since the founding of Jericho. It is still a long time, of course, but with perspective and imagination, it is possible to imagine what it would look like, for example, if a society existed for thousands of years, waiting for a probe to reach a distant star.
There is a direct trajectory between the first urbanization 10,000 years ago, the earliest human civilizations, the birth of modern "science and technology", and what civilizations could look like 10,000 years in the future. Tracing a trajectory between them is hard, but not impossible, and I would like to see more science-fiction that showed how our "distant past" could lead to our "distant future".