There ain't many who aren't of the BC-R type who do that sort of stuff.
Until whomever wants to do some damage shaves off his beard, wears a suit, and says all his prayers before arriving at the airport. Further, "BC-R" might be most of the current threat, but only a little over 20 years ago in the U.K it was the IRA and they used to detonate bombs in and around Britain on almost a monthly basis. FARC and Tamil Tigers also spring to mind as threats that have only diminished in the past few years.
If the risk is that obvious why is it that third party contractors and indeed some foreign owned companies actually screen at Australia's privatised airports?
Economic cost, and (possibly) the ability for an Australian based security company to have the scope, experience, and scalability to provide the level of security required at an airport. I'd need more information about your concerns to answer your question better. But in simple terms Chubb airport security costs a lot less than having the entire organisation run a-la the TSA in the U.S., for example. As far as motivation is concerned, I never seen any evidence that the $25.00 per hour Chubb employee takes his or her job any less seriously than the TSA employee.
DIVOSH!