Why is Australia so good at sports?

The one thing I'd add is that, unlike the US, which either rejected English sports or made up their own versions, Australia took on most English sports. This is significant because many of the Olympic team sports are commonwealth sports.

We then started to embrace US Sports like basketball and, to a lesser extent, baseball in the 20th century.

Part of this is also a lot of people playing two sports, one in summer, one in winter. So people who play cricket in the summer then play field hockey or baseball in the winter. Or they play basketball in summer and AFL in the winter. Then you get to the age to pick a career you take the sport where you can be paid millions a year or get government training and support through the AIS because it's an Olympic sport.