1 problem is that people don't believe it, the second problem is that the rewards for acting are very far off, and even then, it's more like there's no reward, things will simply be less bad now than if we act in a couple of decades.
It's as if a doctor where to tell you now: your arm feels fine sir, but that minor throb you sometimes feel in the tip of your index finger is a disease which in 10 years will cause your arm to fall off. Luckily we can just amputate the finger now and save the arm.
Many people in the above scenario would choose to ignore the doctor, and go on living with the throb, assuming it will never lead to the arm falling off...