They get mad at the very idea that people can work together and successfully create change, despite numerous historical examples. It's actively immature to be wholly cynical

I agree. And I think that cynicism is just easier. The claims of maturity part is mere justification.

I agree with this. People think being pessimistic is more realistic than being optimistic. They think spinning things as negative is automatically more realistic than the positive spin. In reality, realism sees both sides and adjusts one's behaviour to make the best out of everything