Why external auditors just form and express an opinion as to the fairness of financial statements rather than certifying them?

Periodic or specific purpose (ad hoc) audit conducted by external (independent) qualified accountant(s). Its objective is to determine, among other things, whether (1) the accounting records are accurate and complete, (2) prepared in accordance with the provisions of GAAP, and (3) the statements prepared from the accounts present fairly the organization's financial position, and the results of its financial operations. See also internal audit.