According to this YouTube channel that broadcasts science-based educational videos, it is because we humans have the brain of a caveman, we tend to cherry pick, we fear being cast out of our interests groups, we don't perceive future threats as real threats, we don't have space for more worrying and we decide to ignore facts.
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So people who don't believe in Climate change are dumb idiot self-centered cherry pickers with lots of worrying in their minds. People who believe in climate change are the smart, intelligent ones. That's educational science for you.