The I.G.’s handle the investigations of corruption, ethical violations, etc. on the part of upper civil service AND (POLITICALLY) APPOINTED management in the government service. Making them subject to political review in the performance of their (by design) independent performance of their duties.

Also, bringing outside review into play would insure that no one would ever whistle-blow, knowing that their upper management would be cognizant of the whistle-blowing activity, to the detriment of the future of the whistle-blower in government employment.

After 24 years of government service, including serving a a union local president of a federal employees union, this sort of thing would immediately become a critical issue.

Of course if one has no concern with the conduct of upper level government management…or WANTS criticism and reports of malfeasance squelched for political reasons, then hey, this is OK.