If 3 or whatever NAV's in GA is not enough
Who said it wasn't enough? Clearly it is. Pretending that GA has any better CTA training than it has and RA-Aus could never achieve that standard only undermines your argument for no CTA in RA-Aus; you better find a different good reason 'cause this ain't it.
Why can't others do the same?
Because a) there is no good reason to deny it and b) it doesn't make any sense to spend thousands to get the PPL and then never fly a GA aircraft again. All this doesn't seem to be an issue for the sport pilot certificate in the US, but obviously, CASA knows something the FAA doesn't.
I'm glad that the recreational pilots that I share CTA with think like this. It really makes me want to continue to be there, sharing the airspace with you.
You confuse a relaxed state with not taking what you are doing serious. Relax, man. You can always opt not to fly. (The last thing that somewhat raised my stress levels in CTA was two highly trained GA pilots with similar call signs answer calls for the other when the controller was clearly trying to keep one away from me.)