Reasons
Historically spider in europe were thought to carry diseases. During the 14th century the black plague swept across europe killing millions of people. People belived that any food that a spider had touched was infected. spiders were belived to bring death. But actually the plague was spread by infected rats not spiders. In western countries nine out of 10 people who suffer from arachnophobia are women. Of course the tendency of hollywood to make cheap science fiction movie about giant spider does not help those with arachnophobia.