I used to think so too, but evidently weather forecasting is a much harder problem than it seems from the outside. I was talking to a physicist who told me who had first wanted to get into weather modeling, but that it was too hard. I think his quote was something like: "those guys are hard. core."
It's a chaotic system, one could equally well wonder how it's possible at all, even given insane amounts of compute, especially forcasting days and weeks ahead...
