I know that most spiders are not a threat to me, but I still jump involuntarily and get a spike of adrenaline when I notice one crawling around my apartment. The fear is not rational. It is instinctual and very difficult for me to overcome, even though I consciously want to overcome it.
Hypothetically,1 our ancestors were not always afraid of spiders. At some point, a mutation occurred which caused a fear of all spiders, and individuals with this mutation survived to reproduce just a bit more often than those without it — so the mutation spread slowly through the population.
Natural selection doesn’t “care” about rationality, efficiency, or accuracy — it only “cares” about reproduction and survival rates.2 Being afraid of all spiders is irrational, but it works. It significantly lowers your chance of dying from a spider bite.
Evolution works extremely slowly, over thousands or millions of years. In contrast, technology progresses very fast, and is accelerating. This means our environment is changing faster than we can adapt to it — we are living in circumstances we were not “designed” for.