Hold up a ball. Which side points to the earth? What happens when you drop it? Will it always fall towards the earth? Now, go to Australia. Which side of the ball faces the earth? What happens when you drop it? If you drop the ball, will it fall towards the earth? If so, that's why they haven't fallen off. Now of course, move a larger-than-earth, as-dense-as-earth object near the Earth on the side of Australia and your answer will differ. Of course, the Earth is more likely to be destroyed by gravitational tidal forces due to that before you are able to 'fall off' Australia but at least you'd have your tired old joke.