Because nobody gives a crap about Australia. When was the last time they were relevant to anything? Granted, as a gamer-centric website most of us are aware of the great land of Oz solely because of the anti-video game bullshit that goes on over there - but what else?
It'd be like having a FPS set in the Netherlands or Kazakhstan - Not that these countries are particularly bad (Okay, I'm lying: Kazakhstan is a total and complete shit-hole), it's just that places like Australia are a little absent from the world stage.
Take Africa for example: mostly a god-forsaken wasteland, but every modern FPS now has some sort of Africa-based level because of all the war, famine, plague, riots, and commonplace horrible shit that gets everyone's attention.
But I agree; I've been to Oz, and I loved the place. Even that big blank spot you guys call 'The Outback'. I actually think it would be a great setting for open-world games, or hell, anything vehicle-related a-la Mad Max.
* Must note: Even though Fallout was inspired by the Mad Max trilogy, the setting has always been uniquely American. Especially with the first two games of the series: It takes place in an apocalyptic retrofuture that parodies the US world view and futurism of the 1950's era. From the architecture, dialog, and robots to the weapons and clothing, everything was designed to evoke the trends and mannerisms of a time that many in the US still falsely (your mileage may vary) believe to be "the good ol' days" of our past. I could go deeper into what I liked about Fallout 1 & 2, but I just wanted to address the statement on Fallout in the OP.