People are pessimistic precisely because they are aware that if we don't do something about the climate crisis, everything will come crashing down. Being optimistic about it, you might say "don't worry, they'll work out a plan" (whoever they are), and then carry on as before, but being pessimistic, you're more likely to think "we'd better do something, and NOW!". When you realise that Live Aid was one of the earliest…
If most people are pessimistic for that reason, why havent they done anything about it? If you're worried about the zombie apocalypse, you start prepping, you don't start complaining about how rubbish the world is because the apocalypse is coming and no ones prepared for it.
We are facing a collective procrastination of sorts. Just like when you procrastinate, you know very well that you should be doing something else, and you can even be pessimistic about it. Yet you don't.
That's why the governments, people in charge, need to take action, at the very least, start talking about it openly. Then the rest will follow.