That's the point. These potential fans don't go to a Sharks game because no one apart from diehards are travelling to the Shire. Not everyone buys their merch through the club or NRL either. However if you were to take one or two games a year and play them at Allianz, and you get the NRL to schedule it on a Sunday afternoon so fans from the Shire have every chance to attend, then over time you will slowly build it with fans across the entire city.
If the Sharks are content with their fanbase just being people from the Shire, then ignore what i said, but unless Shark Park gets some funding, they are screwed long term and may be forced to move anyway. It's probably the only thing I admire about the AFL, you can have a club like Geelong play at the MCG over 70km away from their home ground and they will still fill it, so clearly they have some fans in Melbourne as well.