Are you suggesting they play the Sydney game on the cricket pitch at the SCG instead? Maybe we could do the same and play our game at the MCG, but with modified rules -- middle of the ground is out of bounds.
They play these regional games firstly for exposure, and secondly because many of the grounds are unavailable because they have a big hard square in the middle of them for cricket season.
They're practice matches. They're not televised, the AFL get little or no revenue from them. They don't care who plays where and nor should they. Neither do I really. Membership promotion is nice, but it's not at all what the practice matches are about. They're essentially just training under real match conditions.