It’s not that there aren’t other reasons. It’s that they use those reasons as justifications for their unhappiness. A “Doomer” would be just as miserable if climate change were solved tomorrow.
I don't want go to back to some mythically awesome time that wasn't really so great. The 50s or 60s (well before I was an adult) - remember endemic racism (far far worse than today), murders of people advocating for civil rights. I want to go forward, building on what we have. I'd like to get to the point that if you have a car accident and can't work your life is not ruined, or you get sick and can't work for a year the same happens. Somehow other western countries are able to pull this off.