Because they come from a different order, Araneae. Like Scorpions and Harvestmen. They are Arthropods that have eight legs, and Chelicerae with fangs that inject venom. They are the largest order of...
An Arachnid is an arthropod with eight or more legs, making the Solifugae (AKA camel spider) an arachnid, but not a "true" spider. Anything in the Arachnid family is considered a spider.