There isn't some sort of magical program where you put in an .apk or .ipa file and it spits out a .cia file.
A game port requires work, and a LOT of it, especially if the game is closed-source. That would require reverse-engineering it, which is an incredibly tedious task to do. And after the reverse-engineer of it is done, the work to actually port it can begin. It's also not an easy task due to the fact that most of the documentation for 3DS development is locked away in Nintendo's developer portal and not readily available.
No one is going to do all of this for free, and ALSO risk a lawsuit from the game developers due to the reverse-engineer.
When someone reverse-engineered GTA 3 and GTA Vice City, it took them years and in the end, Rockstar sued them for it. Do you really think that all this trouble is worth going through just to be able to play Fortnite or Subway Surfers on 3DS?