Why does Australia have so many dangerous animals?

Australia's reputation for lethal wildlife comes largely from its long isolation. The landmass separated from other continents tens of millions of years ago (part of the ancient supercontinent Gondwana), and its plants and animals then evolved in isolation for a very long time, producing a wildly distinctive and separate branch of life, marsupials that raise young in pouches, egg-laying mammals like the platypus, and a great many species found nowhere else. In that separate evolutionary laboratory, some lineages developed extremely potent venoms and defenses. Australia is home to several of the world's most venomous snakes, spiders like the funnel-web, the highly venomous box jellyfish and cone snails in its waters, and other creatures whose toxins rank among the most powerful known. Various theories suggest why the venoms got so strong, competition, prey that evolved resistance driving an arms race, the demands of subduing prey quickly in harsh environments, but the deep answer is that isolation let unusual evolutionary paths run their course undisturbed.

It is worth puncturing the everything in Australia will kill you exaggeration, though, because the actual danger is far lower than the reputation suggests. Despite hosting spectacularly venomous creatures, deaths are rare, Australia has good antivenoms, medical care, and public awareness, and you are statistically far more likely to be harmed by ordinary hazards (or by non-exotic animals like horses and dogs) than by a snake or spider. Many of the scary creatures are shy, avoid humans, or live in places people rarely encounter. So the honest picture: Australia genuinely does have an unusual concentration of highly venomous and unique animals, a real product of its long evolutionary isolation, but the popular image of a continent where every creature is a deadly threat is mostly myth and tourist-scaring fun, and the practical risk to a careful visitor is small.