I really dont know that much about it, but from what I can guess one of the main reasons the climate sims work is because the game knowes where oceans and landmasses are, and knows where the poles are ect. But even on earth this breaks down when you, say, i dont know, flip the earth so much that through the entire year the north pole is facing the south, and the equator will still have no snow, the poles will still be frozen, ect. So really it seems to need more work before they can even think about adding it to other planets
BUT I could also be wrong, so only take my word as speculation.