Simple: Because the theory doesnt stand up to even basic scientific practices. When you're manufacturing data to insert into a model and then assuming the model is true, you're not being a scientist - you're being an idiot. If you or I had pulled that kind of garbage in Junior High biology our teachers would have (correctly) laughed at us and given us a big fat F. Essentially what they are doing with this information is looking at a problem which states X+Y=Z. Z=global warming. X and Y are complete unknowns. If we assume X = human activity and Y = CO2, then we can say Z is caused by human activity. Is this true? No - its pure bullshit.