I've heard the most poisonous snakes on the planet have mouths that are too small to bite us.
Humans are barely a blip on most poisonous snakes' radar screens.
Many of the world's deadliest serpents, for instance, live in remote, unpopulated stretches of the Australian outback. They evolved for millions of years without the threat of marauding sheep ranchers, and thus had no great need to puncture the human epidermis. And their fixed quarter-inch fangs have difficulty penetrating a pair of jeans. Indeed, being wrong would really bite. It's tough to say, especially since most of us don't live according to the severe and unwavering demands of the wild, i.e. we don't need long legs to outstride ill-tempered rhinos, or long arms to reach the upper branches of trees.