[Betelgeuse90]

I think a factor of importance is how much selective pressure there is that could lead to their creation. The sporadic and fairly arbitrary use in humans doesn't create a selective pressure strong enough to create these superbugs in meaningful amounts and have them survive.

When used en masse in large, concentrated populations, you're statistically much more likely to bring these superbugs into existence, and have them survive. After all, an infected human is fairly isolated. But Stick an infected chicken in a hen and you've got some trouble on your hands.