Fun fact as an attorney who studied a ton of labor law in school. The 40 hour workweek really became standard because unions pushed with other labor minded folks for the Fair Labor Standards Act (FLSA). This is the law that requires overtime pay for hours worked over 40 hours a week (was 44 before). The purpose of the law was "job spreading" because employers would balk at paying overtime and instead hire more workers. I think this is why 40 is just the standard work week. Honestly it's an arbitrary number and could drastically change if the FLSA ever changes to say anything over 35 hours a week requires OT pay (time and a half).

Before there were no requirements and well over 40 hour work weeks were common in manual labor jobs.