There are many variables in the process:
- Demand for money of the employee (poor father of five vs. rich single)
- Demand for work of the employer (company needs a helping hand vs company needs to recruit a new team)
- Scale effect of product (does one additional hour or man lead to one additional unit produced?, compare laying bricks vs programming)
These variables are unknown to the lawmaker for each specific instance, so any a-priori determination of the amount of work hours per week cannot be optimal.