Every measurement has inherent errors in it - and those errors are large if the task is to measure the location and velocity of every molecule in the atmosphere.
You also need to measure the exact amount of solar radiation before it hits these molecules (which is impossible, so we assume this is constant depending on latitude and time)
These errors compound (the butterfly effect) which is why we can't get perfect predictions.
This is a limit inherent in physical systems because of physics, not really a data problem.
