Arguably, spiders actually benefit humans by eating other insects that harm us a lot more, like mosquitoes.

It's possible that we've evolved to have an adverse reaction to spiders because at some time in our evolutionary history there were poisonous spiders that caused real harm and we passed on a natural response to avoid ALL spiders like we generally have the same adverse reaction to ALL snakes even though most snakes are not harmful.

It's also possible that spiders are just so biologically different than humans that we have a natural adversion to them. That and they are sneaky, live in dark places and ambush their prey in creepy ways... and like the bats and all the other slimy things that hunt in dark places, we generally avoid all of them.