If your company is causing my company to operate inefficiently and standing in the way of my obligations to my shareholders under the corporations act by providing inadequate facilities then why shouldn't I complain long and hard?
Presumably, the boards at Qantas and Virgin, when actually concentrating on the economics of running their airlines properly, have considered whether there'd be a benefit to trying to take over the airport and build the runway themselves, and have decided against it. No one wants to be left holding the stinky can of debt if the mining boom dries up and the runway isn't needed anymore. It's why only a competent government (hah!) can really be expected to fund this kind of development.