Dave-
Very simply because none of the models has up to date information on the monarch butterfly population in
Mexico.
[That's referring to mathematical chaos theory, where it is said that the actions of one butterfly flapping its wings can ultimately reverberate thousands of miles away and affect what is happening there days later.]
Can you tell exactly what the cloud cover will be tomorrow? Which affects
solar heating on the ground or sea below it, which in turn affects wind formation from the
heating of the air above that ground? No. So, you can't be certain of the outcome three or five or seven days later, the best you can guess if often 48 hours.
I've gone to
bed with forecasts of "5mph easterly winds" and headed out into 4-8' seas and gusts to 40 knots that had half of a
racing fleet cancel before the start. What happened in 12 hours? Damfino.