Is that actually true though?

Maybe Americans get higher incomes because US companies structurally sell to larger markets than Australian companies, so - at least for those roles with scale / leverage - the role is more productive & the absolute value that the employee can extract is high, even if the proportion of the overall value extracted by the worker is similar to or lower than the proportion extracted by an Australian worker.

Maybe if Americans had stronger unions, perhaps American workers could benefit from both higher incomes - due to the increased scale & opportunities of the US market - & increased protections relative to Australian workers.

Another thing that is quite different between the US & Australia is that in the US, the public is generally a lot more trusting of business over government. US business interests have a long history of influencing US public opinion- there's examples of stuff like business interest groups creating educational materials & getting them into schools, so kids learn early about the wonders of capitalism & free markets. It's unsurprising that you might not find unions so popular in such an environment. A book that details quite a lot of that history is Kerryn Higgs' book Collision Course: Endless Growth on a Finite Planet [1].

[1] https://direct.mit.edu/books/book/3683/Collision-CourseEndle...

They would not. If its harder to fire people, then there would be less employee turnover, which is where all the income growth comes from.