Why are pilots not able to certify documents..........
Have you applied to be appointed a Commissioner for Declarations or Justice of the Peace in your home State? I am aware of a number of pilots that are appointed JP or Com Dec.
Being able to certify documents has nothing at all to do with being " trustworthy people". NSW Police and Federal Labor Politicians are able to certify documents............
Certain professions are able to certify a limited range of documents because they are identifyable within their profession, usually due to their professional registration. (And before you argue that you have an ARN, the medical profession registration is publicly available, where as a pilot's ARN is suject to privacy legislation.)