Because a CO2-to-Global Temperature R^2 correlation of 0.44 is not good.
CO2 is not a greenhouse gas.
Pre-historic global warming trends have also been associated with increases in CO2 levels. Since the dawn of the Industrial Revolution, man has been pumping CO2 into the atmosphere at an unprecedented scale! http://climate.nasa.gov/key_indicators#globalTemp
There is enough scientific evidence and reasoning to support man-made changes that would be highly improbable to occur without our influence, yet much of the change is also natural and irrelevant to human beings' actions and carelessness. I conclude with man-made because I believe that aspect ought to be acknowledged with explanation of the obvious co-factor (nature).