Because I don't believe people and businesses can accomplish the individual environmental goals, much less fast enough, on their own. If human contribution to climate change is a result of aggregate behavior, then we're talking about drastically changing the micro behaviors (what you can eat, how you travel, no black markets, etc etc) of everyone on earth. I believe that can only be accomplished through extreme authoritarian control. As people start realizing they can't do it on their own and the various climate deadlines approach and pass, I believe they'll be more pliable to extreme forms of government.

>but devil's advocate - is fascism worse than literal extinction?

That's the question everyone will have to answer, and it probably boils down to the core of that person's philosophy. People who say "no" though should really consider all of the ugly side-effects that result from all-powerful authoritarian entities being able to control lives on a micro level.