That's really not true. Yes, auditors do find missing assets like that, but some of these banks have trillions in assets. Moreover, these errors usually aren't things like cash in versus cash out. Accounting and tax rules are very complex. So an error might entail having incorrectly computed the value of certain tax assets or the like. More to the point, a major bank will have layers and layers of accounting controls intended to catch even very small errors. These processes are themselves audited every year, and stress tested. It's not duct tape and excel spreadsheets.