Sampling misses what repeats
A person checks what there is time for: large amounts, familiar problem items, totals. A deviation within tolerance gets waved through — correctly, each time. The same deviation repeating week after week is a process defect that costs money continuously, and a sample cannot see repetition: it has no memory of yesterday. The agent checks every operation and keeps the history, so a systemic violation becomes visible while it is still systemic, not when it has become a year-end write-off.