Human inspectors have several drawbacks. One is we tend to be expensive. Another is that no matter how dedicated we are, few of us can maintain the concentration necessary to perform such a boring task for extended time periods.
Production line optimisation in modern century define by human not by machine
Humans are by nature variable. Even with the best controls, an individual operator can have a bad day and introduce variations from one day to the next. Two different operators trained in the same way might have slightly different actions or criteria for decision making, which causes variation.