Just like in infosec, controls (etc) do not guarantee safety (accuracy), but they let you know that processes were followed and that fact is documented. Third parties (accounting firms) confirm that you have the evidence of having followed your processes. Corruptable? Sure. Better? Definitely yes.
Just like in infosec: if you have evidence that every week you've plowed through the access logs, you're more likely to have caught an intruder/mistake. The approach tends to route out single acts of sloppiness and subterfuge and turn mistakes in to conspiracies, which is a much harder thing to pull off than a single actor looking for fame or a raise.
For my part, I genuinely believe these organizations are trying their best to do something very hard but that their own efforts at fairness can be undermined by a lot of factors, especially money. The natural incentives (clicks/$) need to be counterbalanced with self-imposed "regulation" that is third-party verified.
[EDIT: s/factories/factors