Sci-fi is something that in RL, right now is technologically not possibile, because we have no idea how to build that tech, but in theory it doesn't violate the physics rules or at least is strictly coherent with its own physics rules, and these physics rules aren't stated impossible to exist in a different reality.
Fantasy is, instead, something that it is already stated impossible also in theory that it violates its own physics rules, like magic, indeed, fantasy genre is usually associated to the use of magic, but actually it isn't just the use of magic, in general, as an example, the use of any deus-ex machina is fantasy, even in a futuristic setting, read it as quite infinite thrust/weight, read it as unmetered fuel consumption to gain that thrust/weight ratio and so on...
Just as a loose reference: https://www.nownovel.com/blog/difference-fantasy-science-fiction/
EDIT: It's pretty easy to note that is by far much harder to write a true Sci-fi space simulator instead of an ostensible Sci-fi but actually Fantasy one. Worse I am pretty confident that a true Sci-fi space simulator means a much more niche player base in comparison with a Fantasy one...
"Any sufficiently advanced technology is indistinguishable from magic." - Arthur C. Clarke