Why do people have arachnophobia?
Some reasons why someone may be arachnophobic:
- Misunderstanding of how harmless these creatures can actually be. Most arachnids are quite small and don't pose a significant threat to humans, unless handled or consumed in large quantities. For example, tarantulas can deliver an incredibly painful bite but their leg span is typically no bigger than the width of your palm. So if you're worried about getting bitten by a spider it's probably not rational, even though there is a very slight chance that could happen. 
- Inability to separate fact from fiction when it comes to arachnids. People might hear stories about huge spiders hiding in dark corners waiting for someone to stumble upon them so they can pounce on them, which would certainly cause anxiety. But such stories often bear little relation to reality. While some species of larger spiders will occasionally hunt other animals for food, most adult spiders live solitarily, eating small insects. Even big jumping spiders like the Golden orb weaver only reach up to 7cm (2.75in) long - hardly enough to threaten a human. The vast majority of spiders aren't dangerous to humans at all, let alone able to jump out of their hideaways and grab them in one bound.

In conclusion, arachnphobia has more to do with irrational fears and misunderstandings rather than actual danger posed by any specific type of arachnid.