Basically they're doing a utilitarian calculus. But since our calculus is basically all pretend, we can assign arbitrary importance to anything we want just by daydreaming the costs and percentages of these super unlikely, super catastrophic events.
This is not unusual - it's a core symptom of various mental health problems. There's even a fairy tale about this - a family of foolish people who catastrophise so much that they are unable to accomplish anything [1]. People assigning bizarre weights to unlikely probabilities is very human, and problematic regardless of ethics.
I may, of course, suffer from the very problem you're describing.