The easy answer to this question is that advertisers do this because it helps them to sell their products. Advertisers will not want to produce advertisements that do not sell. If they are producing ads with aspects of Australian identity in them, it is because they feel that those ads will help to sell their product.
The more interesting question, then, is why using aspects of Australian identity helps to sell products. The most likely reason for this is that Australians are proud of their identity. It is not surprising that this should be so. Australia is a prosperous country with a thriving democracy. It has been a country that has accepted people of many different nationalities and made them all into Australians. It has a clear identity that sets it apart in at least some ways from other countries of the world and, in many ways, from its neighbors.
It is for these reasons that advertisers use aspects of Australian identity in their ads. They know that Australians have a great deal of pride in their country’s accomplishments and in its unique characteristics. They know that appealing to the pride will help them to sell products.