I think it has to do with how isolated Oz has been in human terms for so long. We've been bitten and poisoned a lot in the populated areas that statistically people survived them and passed along their resistant genes to future generations and collectively built up a sort of tolerance. We've also been exposed long enough to work on antidotes and solutions. So what we encounter in Oz is relatively new by comparison--and why it's so deadly.
Which brings up how scary it could be if we unfreeze some ancient organism or something from the arctic or encounter one from space!