it's because it is actually too late to do anything. If all human carbon emissions stopped tomorrow, the reef would still die. It would take 20 years or more for the environment to be able to support reef life again. At that point it might be a wonderful ground for a new reef, but there's no way the old one can survive that long. Anyway, that will only happen if carbon emissions hit zero tomorrow, which is obviously ludicrously implausible.