Because all the climate models say that it's a terrible idea to let it completely loose. They even predict that it's a pretty bad idea not to do a lot more than

Because all the climate models say that it's a terrible idea to let it completely loose. They even predict that it's a pretty bad idea not to do a lot more than what we are doing now. Given the current state of matters, whatever we do it will be at least in part preparation for the changed climate and in part preventing getting things worse.