Not to mention other issues like crewing, etc. The crew that arrives in LAX will normally depart two days later back to AU. If you add in a return LAX-ORD-LAX (two days later) then they wouldn't be returning to AU until five days later. This then means that a five day trip turns into an eight day trip, so eight crews to operate a route instead of five. The revenue from operating these flights would not cover the extra costs - airways fees, fuel, staffing costs - so it is indeed better to just leave the aircraft sitting there idle.

Dave