Audit fieldwork is often done by junior staff who aren’t qualified accountants. Junior staff may not have the confidence to flag or query issues with clients and thus re-select their samples or falsify audit schedules for an easy life. Might not happen often, but the truth is, it does happen sometimes.
Auditors are watchdogs not bloodhounds, and mostly they do a very fine job. And companies must shoulder their share of the burden too. An auditor’s role is not to detect fraud; ultimately it is to ensure that financial statements are materially free from error. Most of the time, they achieve that, and auditors fulfil their necessary role in the financial system.