Because you don't have mechanics or chuck in his pickup doing work down there he is not "authorised". You have approved facilities, engineers engine airframe, avionics radio etc, lots of systems and paperwork ( do you wipe your bum before or after the toilet better check the procedure and fill in the forms ) blah blah blah. It has been over complicated by your regulator and the result is no better and a lot more expensive.
How to make it complicated and hard do it in Australia.