For the same reason that we can predict how a large body of mass will react to a collision but can't explain how the subatomic particles will react to that same

For the same reason that we can predict how a large body of mass will react to a collision but can't explain how the subatomic particles will react to that same event, or the overall trend of the stock market without being able to predict the behavior of any individual stock. Scale matters. Most climate prediction software looks at weather on a far vaster, long-term scale than hours, days, or even weeks. Variations over such short periods ultimately have little effect on long-term trends.