It's actually the opposite. Calculating an asteroid impact is much easier because it primarily involves basic Newtonian physics. In contrast, the climate is a chaotic system, making long-term predictions far more complex.
I don't know how it's opposite. There's some complex guesstimating on where an asteroid will hit. We roughly know its size. We roughly know its composition. We roughly know its speed. We can give a +/- range where the impact area will be. That's not any different than how hurricane spaghetti models do.
The difference is knowing the size, speed and composition of a hurtling rock tells you where it's going to go, where as knowing the size, speed, and composition of a hurricane does not tell you where the hurricane will go. Complexity isn't a problem, the issue with weather prediction is that it's chaotic - extremely small changes to starting conditions cause large changes to the end result.
