It's a 30 minute journey on the inner airport road at the speed limit from QF domestic to international in a car. This transit time between would be a considerable commercial penalty for QF. They would want to make it easy to manage delays with complexing domestic to international just by walking a few hundred metres rather than bussing over the other side. Making it seamless also makes travellers sticky to QF international.
It also cuts Perth Airport out of a revenue stream with passenger International handling charges for their terminal and duty free sales which is probably where the issue is with PAPL.