> As someone who knows nothing about this area, I don't understand why audits won't always detect fraud.

as some one who studied accounting and auditing, here is a page from my text:

https://kfknowledgebank.kaplan.co.uk/audit-and-assurance/aud...

but the tl;dr is that auditors don't provide "insurance", they provide "assurance", specifically reasonable assurance.... that the accounts are "true and fair"

or to be put it in even simpler terms, they can't guarantee something fishy did or didn't happen, the transaction scope is just too much, they will "try their best" and do enough of a check to say if anything fishy pops put.

    > Is it just a top-level glance at the numbers because there isn't enough time/money to scrutinize everything?

yes you hit the nail right on the head. Of course things have changed, govt have put their own requirements in addition to auditing standards, but still that's an adequate summary.

the more through of a check, the more difficult, time consuming and expensive it becomes, and at some point the fraud becomes cheaper than the audit.

but even more importantly is the mentality. There is a phrase we were taught "Auditor is a watchdog and not a bloodhound" that kind of explains what auditors are supposed to do.

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i left the field but i'll try to answer to the best of my ability