Because the venom is composed of polypeptide toxins they are digested. They would half to be absorbed undamaged from the alimentary canal to the bloodstream to exert their toxic effect
In addition to Rabelais' excellent answer, the venom is contained in small sacs just behind and above the fangs in a well protected area of the bony skull. The animal would have to eat that specific area before coming into contact with the venom...