Because the voting system is only one of many factors that determine what kind of government you end up with. Who controls the money in Australia? Who would they prefer to have in government representing them? Are there any ways they might use their money to propagandize or lobby for favorable laws?

(I don't know the Australian answer to that, but where I live the answer is "the ultra-rich and the investor class", "Republicans, but they like to keep Democrats in their pocket, too", and "yes, and it seems to get worse every year")

RCV would be a great improvement, but as long as money rules, things are gonna keep sucking.