Why are so many people afraid of insects?

I am personally a HUGE insect freak. I love them so much, and yet I see people around me and they al hate them. If I show my tarantula to my cat she doesn't care. If I show my tarantula to my friend, she freaks out. I don;t really understand why humans have to be so afraid. I'm sure you've never heard of a cat or dog with arachnophobia...