Take the Box Jelly Fish for an example. The worlds quickest killer by way of venom. Why does it need such a potent venom? The answer....
Because it is a fragile creature and a stugling prey animal would do it sever damage. Makes sense to me anyway.
As to the funnel web. Australia does not have native populations of large predators (outside of that extinct marsupial, the Taz-Tiger). It seems to me that the only real threat to its life (besides some lizards and other spiders) is getting crushed by large animals. And its only defense it that is to announce its presence and to act agressively. At least that is my off the top of my head response.
EBUCK
The problem is that the funnelweb's poison seems to be specific to humans and monkeys, far from the biggest problem in Australia. Humans naturally avoid big spiders and I'd guess monkeys do. You'd figure if the thing were trying to "evolve" a way to keep from getting stepped on or hopped on by larger animals, the venom would be specific to kangaroos...