Science-fiction, save for a few hard SF novels which are too boring to even contemplate, is straight-up fantasy with just a slightly different cover.
Asking why fantasy is more popular than science-fiction is like asking why sports is more popular than football.
You're surreptitiously redefining "fantasy" as "not-realistic" instead of looking at what there actually is. SF is supposed to be realistic. Fantasy is not. Realism as a mode of literature rather presupposed that the world made sense (another Enlightenment idea) while fantasy doesn't, whereas SF does. Fantasy can be serious (not necessarily solemn, indeed the manifest nonsense is rather discordant with solemnity) but it is not pseudorealistic.
Further, the SF=fantasy hypothesis fails the acid test from real live readers. Kage Baker, Catherine Asaro, Justina Robson and Sandra Macdonald write SF/romance hybrids. Fantasy/romance hybrids are written by authors as diverse as Charlaine Harris, Sherrilyn Kenyon, Laurel K. Hamilton and Diana Gabaldon. The readers can tell the difference, and they don't like SF. You may be trying to say the difference is stylistic, but in literature style is hugely imnportant. This is true even if you don't hold to English lit cliches about what's good style.
PS So-called hard SF can be quite fascinating if you've ever taken an interest in the world around you. Not one hard SF novel is a textbook.