Because the system is chaotic and we can't predict it beyond a week. Maybe a day at best. Once you can predict hurricanes, volcanoes, and tropical storms, then let me know.
You're confusing weather and climate. We can't model the weather very well. It's hard. We can model the climate reasonably well (our models fit historical data really well), but the models turn out to be bad at predicting the future because we underestimate how much of an impact we're having (the article is a good example).