Because they feed on it! Just kidding — actually, fear just makes spiders look bigger, as scientists determined when they had a bunch of volunteers hang out with tarantulas and then guess their size. People who were afraid of spiders estimated them as bigger than did those who were just fine with the eight-legged guys. And overestimating their size might have made the subjects more afraid, leading to a terrible spiral of panic and doom that leads directly into the maw of Shelob. Time to despair.