Because they're DEAD. As of them dying, they're just a large pile of Bacteria food. After a while, about five or so munutes, all of the body's systems needed to keep it alive (i.e. Cardiovascular System, Nervous System, etc.) lose all hope of being restarted, because at least some of it has been eaten by bacteria. Even if there was a "Zombie Virus", it would have to be pretty complex in order to take control of even the person's instincts.