I fully understand what you're saying, but I think you've got me wrong.
If you're a huge company, do auditors go over every single invoice, even those in the petty cash for bog rolls & biscuits?
We have independent auditors who work with finance to go over everything & highlight any discrepancies.
My comment about relying on supplied data goes to issues like those affecting Donald tRump.
When years long fraud was uncovered, Mazars confirmed they were acting on information received & requested. They subsequently dropped him as a client & testified against him in court.
As a CPA you'll appreciate cooking the books can easily be done, especially when dealing with paper trails. Even in the days of electronic entries, where there's a will, there's a way, hence why there's an element of trust.