Very insightful question. As you probably know, some of the creations of H.P. Lovecraft are far more alien than most fantasy creatures—yet in an unsettling way, far more familiar and threatening. As to your question, if we did not have six and eight legged creatures in reality, then whenever they were depicted in fantasy we’d react like we do to the six-armed Vishnu. It would just look “wrong” but not threatening as a Spider, Crab, or Face Hugger.