Autism

Evolutionary psychology is behind one of the primary theories, in the simple way that it’s prudent for us to be wary of things that might hurt us. Bugs can sting, bite, invade, cause sickness/death, or poison us.

There’s also the disgust aversion angle, which is tied to the relationship between a lot of bugs and indirect environmental threats (also ties into evolutionary reasoning) like rotting things, or corpses specifically.

In the broadest strokes, we associate bugs with pain, disease, death, and decay.

Arachnids also include scorpians and ticks, so spiders it makes sense humans evolved that way. Perhaps some proto-spider was a lot more dangerous.

Though, jumping spiders are pretty chill and what got me to be less afraid of spiders. They're tiny, they're adorable, they're really friendly, and for some reason they didn't trigger the same arachnophobic response in me. I have a theory that perhaps jumping spiders fed on ticks and other bugs that ancestors of ours might've had, and so we became less afraid of them. Spiders cooperating with other species isn't new, such as the dotted humming frog.

It is! Bugs spread disease, and humans that kept away from bugs got less sick, so evolution being what it is, we are now afrade of bugs.