I agree that the AFL don't have as many options to expand to as the NRL, but as mentioned they have just added another two teams over the last couple of years.
They could have moved out two of the smaller Melbourne teams to accommodate these two new teams, but instead they continue to pump in many millions a year to keep them afloat, in addition to the tens of millions a year they spend on the Giants & Suns.
My point is that they wouldn't be peeing away these millions if there wasn't a benefit, and I think the main drivers are to keep their stranglehold on Melbourne, and they want to avoid the negative look of upsetting thousands of people when their hundred year old club folds/ moves.
They obviously think its better to spend the millions a year to keep everyone happy.