Because there's not enough evidence to make that claim. The structure of indigenous societies were nomadic hunter gatherers who were (as far as the evidence tells us) subsistence hunters. Truth is Indigenous Australians didn't have the population (or population density), nor the culture to hunt these creatures to extinction. Being largely nomadic within set ranges (i.e they travelled often but within their own nations land) it was suicidal to overhunt and deplete the resources, and this becomes a stronger point when you consider the timeframes on which the Aborigines existed on their land (again 20,000 years of overlap here). Also consider the sheer number of creatures that went extinct here, everything from Marsupial Lions to gigantic emus and echidnas, it doesn't match up with indigenous australians hunting behaviours that somehow they decided to overkill for this short period of time all these larger creatures, and then that behaviour suddenly stopped? (we have no evidence of Indigenous Australian's overhunting any other creatures to extinction).
To go even further, there's nothing in their oral history or paintings which gives the slightest hint that overhunting or hunting for status was ever present in any Indigenous Australian culture. Considering the ice age ended around the same time as the death of the last megafauna in Australia, I think its fair to evaluate every conclusion you make about the death of the megafauna within that context.
I'm not saying Indigenous Australians didn't have an effect on the death of the megafauna, anything being hunted in a time like a massive climate shift is going to affect population, but there's no direct evidence, nor indirect evidence about the behaviour and cultures of Indigenous Australia which point to mankind being the driver behind the numerous extinctions.