Yes they can be registered on the VH register but then the users and maintenance upkeep must jump through the more stringent and more costly hoops. As XXX said its not that I think I'm amazing with my PPL and my F18 (if you can tell me where I left it, it would be much appreciated) but an issue of safety.
It is exactly like a go-kart, they perform over 100km/h are driven by people who don't fit the bill for a road license (can be driven by people under 17 for example) and is by all accounts a skill that would probably allow you to drive on the road ok. But that's not the process, the process is to get a road license so we are all judged safe by the same yardstick.