I think one thing that in theory should make Sci fi lead to more differences, is that different planets/ moons etc would have different qualities which would lead to life developing quite differently, both physically and mentally (I liked the three body problems glimpse at sentient life on a planet which will have cataclysms resetting things) whereas all life on one planet could lead to a bit more uniformity, when everyone faces same gravity, day / night cycle, seasons etc.
Of course some of the biggest challenges we have is that if we go down road of trying to make things comprehensible for readers/ players of fantasy or sci fi, it necessarily gets colored by what humans can comprehend / imagine, we have views of what immortality could do to a species, but is formed from our basis of understanding our life spans, and immortals may have very different views.
Of course, doesn't mean we shouldn't try to come up with differences etc that we can imagine, and I think a difference between literature and a role playing game, is that literature only needs enough differentiation to support the sort of story it is telling, whereas a role playing game to me should strive for ways to show q lot of differences, whether through mechanics, traits, descriptors etc to allow for roleplaying of those differences.