Correct, for auditing cash balance, there are many simple procedures, but a financial statement audit opinion is not about one balance on the financial statements. Getting an "unqualified" financial statement audit opinion issued requires the auditor to independently obtain reasonable assurance the financials as a whole are in all material respects in conformity with accounting principles generally accepted in the relevant jurisdiction.

So, an agreed upon procedure approach might allow a company to obtain a tailored report from an independent party which will just focus on cash balance. It might even end up being the same procedures that might have been performed under a full scale audit scenario (bank confirms as you noted).