Not trying to seem sound or be racist. how did people whom technology wise were are par with the Native Americas Incas and Mayas all achieving building great monuments with little to no metal tools. the boomerang itself needs some complex aerodynamics to work. That to me rule out a stick being thrown and it just caught the wind right for them to refine it from there. the fire piston work by rapidly compression of air to get hot fast but need the right compression ratio. How could tribal people just come up with these simple yet complex tools.
I'm afraid I can't answer your main question, but I will say this. The Australian aborigines were a lot more technologically sophisticated than they're commonly given credit for. The tool use displayed in creating an array of very clever fishing hooks is quite impressive and they actually utilised some quite advanced boat building techniques at one point. As to why they never built any large permanent structures, that had less to do with resources and more to do with the nature of their society. They were almost entirely nomadic, so they're tribal groups never stayed geographical static long enough to build anything like that. The other cultures you mention were stable agrarian societies, largely because because they had pastoral land suitable for agriculture and mega fauna that could be easily farmed or hunted. Conditions like these allow for more stratified societies where specialisation can occur. They give rise to separate groups of farmers, warrior casts, bureaucrats etc... Australia's environment is quite unsuited to this type of development, as shown by the horrific consequences of creating grazing land and large scale planting of grain crops by white settlers. Australia has very thin top soil because of its lack of tectonic activity. Farming has had a devastating effect on the environment due to soil erosion. But I digress... No large scale farming + roaming hunter gathering culture = no large permanent man made structures. No idea how they invented the boomerang though, those things are fucking lethal and incredibly clever, they don't seem to have an equivalent in any other culture. Bit of a head scratcher that one...
First of all, as /u/Iota_Korp noted, one shouldn't confuse lack of materials with lack of intelligence.
Second, they had a lot of time to tinker with the designs, and it could be said that the boomerang and fire piston "evolved" through a cycle of variation, selection and retention over the centuries and millennia. It seems that boomerang for example began to evolve from a simple throwing stick that has been used in hunting since time immemorial; for one example, see this Egyptian tomb painting:
It is conceivable and in my personal opinion highly likely, although hard to prove without shadow of doubt, that over the years hunters noted that some throwing sticks worked better than others, were more accurate, or might even begin to claw back to the thrower if they missed. (If you spend years and years hunting with one of these, you begin to notice the differences fairly soon, I'm sure :)). Over the years, these incremental improvements resulted to the forms seen today - and if I were to guess, this progress was surprisingly rapid, possibly happening within a few generations of hunters.
In fact, Aborigine boomerangs and other throwing weapons featured prominently in a (somewhat outdated) but interesting early theory of technological evolution put forward by wonderfully named Victorian British general Augustus Henry Pitt-Rivers in 1906. Pitt-Rivers was deeply influenced by Darwin's Origin of Species and Spencer's (flawed) assertion that the entire history of life was marked by a development from the simple to the complex, and resolved to apply similar approach to technology. He collected series of artifacts from around the world, and arranged them in "evolutionary" sequences beginning from what to him seemed the most simple tool or weapon. [1] The resulting arrangement of Aborigine boomerangs can be seen in the picture here (the link is to Pitt-Rivers' 1906 lecture, later published in a book Evolution of Culture).
Now it is important to note that this approach is today considered somewhat flawed. The artifacts were all in use by the time Pitt-Rivers collected them; they do not represent a time sequence, and Pitt-Rivers didn't even bother trying to date their appearances in archeological finds for example. The mere existence of simpler models of the artifact - particularly as they were in simultaneous use - does not prove that such evolution happened. There are also critiques that reject the idea of unilinear "progress" as hallmark of human culture altogether [1]. However, Pitt-Rivers himself was careful not to make too sweeping claims, and his most fundamental point - that technological change occurs not in a series of great leaps but in modifying artifacts already existing - is still largely valid, and in my opinion easily explains the boomerang and the fire piston.
In fact, this approach has seen increased interest lately, for example in the so-called "recombinative" or "cumulative" models of technological change. A very good read to anyone interested in the nature of technology and technological change, and the evolutionary model, is Brian Arthur's book The Nature of Technology. [2]
[1] Basalla, G. (1989). The Evolution of Technology. Cambridge, UK: Cambridge University Press. Pages 17-21 give an overview of Pitt-Rivers's work.
[2] Arthur, B. W. (2009). The Nature of Technology: What it is and how it evolves. New York: Free Press.
TL;DR: Cumulative "evolutionary" improvement based on variation, selection and retention over the years gives great results in biology, so why not in technology?
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