You are right that there are non-stoppers from east-Oz capitals to USA's west coast airports as well as a couple of internal US domestic hubs, but I think getting from Oz to the USA's east coast attractions takes a hop somewhere, and it may as well be China as anywhere else. I'm not keen, for example, to jump off a 14-17 hr full-service international long-haul to jump into a US domestic internal carrier for the last five or so air-hours.
I think most of Oz's inter-continental long-haul travel is to Europe rather than the Americas anyway, meaning we head west rather than east for preference. I think once the big Chinese airlines get themselves plugged into the Oz market, and given the opportunities provided by routes over the pole from there, we might head north regardless of our northern hemisphere destination.
It's a bloody long way to just about anywhere else from here, so if you have to go halfway around the globe to get there, it isn't very material which way you go
I flew last into and out of Philadelphia by Qatar through Doha, and could have reached any number of US and Canadian eastern cities that way - much more fun than transiting through LAX and then picking up an internal.