Because that way a lot of money gets spent with virtually no outcome in the form of useable or deployable capability, that’s why. Ares units with modern equipment do not provide a modern warfighting capability, not without extended full time, lead up training, additional manning resources, fulltime service agreements and then they are ARA in all but unit designation anyway.The ARA is built around the idea of supporting one expeditionary combat brigade overseas. Australia has not been able to do so for quite some time (we needed US, Kiwi and CMF attachments in Vietnam). With the new kit and bodies coming over the next few years we may well be able to sustain a modern high intensity brigade strength warfighting capability. This is, however, not the same as defending Australia - we couldn't afford the large permanent standing Army necessary to do so. The Army Reserve gave us a cheap, quick, surge capability. I am not sure why this role is now disparaged.
Australia does not have the resources available to properly equip all of Army to the same standard and provide actual warfighting capability from all it‘s units. Attempting to do so, didn’t deliver the capability needed from Ares and diluted capability within ARA because resources were diverted away from them. It was a neither fish, nor fowl approach.
Unfortunately, as @Takao and @Raven22 have pointed out, 2 Division appears to still not have properly adjusted to this reality and worked out a decent role for itself, within it’s means (not just cash...) that can actually contribute effectively to overall capability.