Good question....I have wondered this too....
If one takes venom of deadliest king cobra orally, he won't die (I have seen seasoned snake charmers sucking from the punctured twin holes on the skin of the victims to take venom out) and never seen any snake charmer lost his life. This is the same venom, if mixed with blood stream by snake bite, death is confirmed in a couple of hours.
Without going into hardcore scientific deatils (as we are not experts to that extent), route is a key factor for effectiveness. Same applies to HIV, which is the most fragile specie outside the body, but the strongest virus known to mankind to date, when it is residing inside the body.
Why to waste time on reinvent the wheel, better to use the wheel for our benefit