A Definition of Science Fiction

Why is Science Fiction (sf) so notoriously hard to define?

Merriam-Webster defines sf as "fiction based on imagined future scientific or technological advances and major social or environmental changes, frequently portraying space or time travel and life on other planets." --but this does not quite cover all the bases, does it? What about sf that happens in the past, like Star Wars? Or sf that is not based in scientific inquiry or technology, such as the novels, A Canticle of Lebowitz, or The Road? What about sf stories that do not employ time travel, aliens, or spaceships, such as the famous short story...