Problem with that, at least for Bankstown, is that private GA is very expensive and Bankstown and the surrounds are an area of average to below average wealth with typical households most likely being unable to afford to spend money on private GA.
For those that can afford it, it’s a time consuming commute from the more affluent areas of Sydney into Bankstown airport. I was involved for in GA there for a bit as a hobby but the travel from the north shore was too much of a pain to continue. Sitting in traffic for a few hours to do a 40 min joy flight became too tedious (and forget about trying public transport!)