Cloud formation is affected by cosmic ray flux. It's effectively random.
But the real problem is chaos - which says that even with perfect data, unless you also have computations with infinite precision and time/spatial/temperature/pressure/etc resolution, eventually you wind up far from reality.
The use of ensembles reduces the effect of chaos a bit, although they tend to smooth it out - so your broad pattern 12 days out might be more accurately forecast than without them, but the weather at your house may not be.
Iterative DL models tend to smooth it faster, according to a recent paper.
It's the problem that's hard.