Yes those are harder to find, because they are much harder and more time intensive to produce and therefore would be more expensive and harder to sell.
Sci-fi is harder to combine with different settings. A lot of fantasy things also work much better with slight adjustments in different settings, you can do aventure settings, pirate settings, gothic horror settings, realistic historical settings and a whole lot more with it, while sci-fi let's you do nearly exclusively sci-fi.
And from a production perspective I am always trying to do new things which work together with some other assetpacks I already produced.
Maybe in future I will end up doing some things in that direction.