the answer is simple.

its a reaction developed on the genetic level. you know how prey is frightened of its predators? well they're frightened because through their evolution the genetic mutation that provoked that response was selected for via natural selection. the same reason for why this response was selected for applies to humans being frightened of spiders. only its not nearly as strong for us because we havent been killed by spiders nearly as often as prey is killed by predators. this seems to have manifested itself in mild caution in some and severe fright in others and everything in between in most.

if spiders and like creatures were not venomous to humans we would think entirely differently of them.