Why is Australia's highest mountain not even half the height of the highest peaks on other continents?

Answer 1

The older mountains are, the more they will have eroded. Australia is the Earth's oldest continent, so its mountains have had more time to erode than most other mountains have. Many other mountains ranges, including the Himalayas, are much younger than those in Australia, so they have not eroded nearly as much as Australia's mountains, and are therefore higher.