I could get a CA/GRO certification tomorrow, front up at my local aerodrome, sell myself and the service I could provide, get a job, and away I go.
Things aren't Mickey Mouse with a CA/GRS as you seem to think.
There is CASA certification and approval required for both an AD OPR and CA/GRS provider, requirement for operations manuals by both, safety case and ongoing CASA oversight involved.
There is also the little matter of indemnity insurance in case something goes wrong .....
At some locations there is a need for a local service between nothing - Class G - and full CTA with a TWR, with its associated $M construction, staffing and running costs.
UNICOM and CA/GRS fill that gap.