That's a perspective on Mary Shelley that I hadn't considered. But she was reasonably well-adjusted and popular. And yes I do consider Frankenstein to be the first English science fiction.
I don't refer to mary shelly. I do not distinguish her as the "inventor" of science fiction either. Rendering strange ideas in terms of esoteric disciplines for the metaphorical augmentation or whatever is as old as humanity.
Good question! Typically they get listed as fantasy because the magic isn't manmade. Most definitions of science fiction require a human to have created the unrealistic element - or an extraterrestrial lifeform who is roughly analogous to a person. It's not just that magic is present, but that it was derived from supernatural sources and not by human actions.