Because nobody listens to the scientists. You have two sides where one side thinks we should sacrifice (all) economic growth for the sake of the planet and another that thinks we should sacrifice the (entire) planet for economic growth. The future will hopefully look nothing like either of them. We should find a healthy balance between the two and that means we need a self regulating system and preferably it should resemble something we already have. Landfills charge for disposing of garbage so why not charge for atmospheric pollution? It already exists and is called the Kyoto Protocol.