One of the reason is "piecework". It is any kind of work in which a laborer is paid a settled piece rate for every unit delivered or activity performed paying little heed to time.
In the US, the Fair Labor Standards Act requires that all workers, including piecework representatives, win in any event the lowest pay permitted by law. In figuring a fitting piecework rate, businesses must monitor normal generation rates for particular exercises and set a piecework rate that guarantees that all laborers can win the lowest pay permitted by law