The answer is obvious, missy: There must be profound safety consequences of crossing that line. I feel that 'chill' and the shudder in the airframe each time I cross.
For those who aren't across the facts underlying the comment I made and missy's question, this is from the ATSB report on the Mangalore tragedy:
In the Australian ATC system STCAs occur in both controlled and non-controlled airspace, with alerts inhibited in some areas. Specifically, Airservices advised that STCAs in Class G airspace are inhibited below 4,500 ft in the Brisbane flight information region, but occur to the ground in areas of the Melbourne flight information region.
The real answer to missy's question is the one of which I suspect missy is aware: Decades of, first, Airservices being left to its own devices as a consequence of the combination of CASA's inadequate regulation of the ANSP and airspace arrangements and, secondly, ATSB's inadequate investigations of the safety implications of the first.