I am not defending the IG. From what I have seen they are worthless, except as you indicate, to investigate and then DO NOTHING. They can get away with doing nothing because they are independent of the political control system. It's not constitutional as they are executive agencies outside the control of the President.
But presently they are independent entitities.
I guess it all depends on what the meaning of the word ‘independent’ is. How often do IG’s release reports that fully expose serious malfeasance and mismanagement within an agency and how accurately do they report the true financial cost to taxpayers? The answer is never.
Another factor in the world of IG’s is congressional pressures. Yes, there is that too. Trust me, it happens.
If they were truly independent what would stop them from aggressively investigating wrongdoing and letting the chips fall where they may?