Because the marginal cost, to the employer, of hours beyond 40 is zero – remember these employees are almost always paid a salary, i.e. a fixed amount regardles

Because the marginal cost, to the employer, of hours beyond 40 is zero – remember these employees are almost always paid a salary, i.e. a fixed amount regardless of hours worked (beyond 40). Thus, even if an employee's productivity plummets – but is still above zero – it's in the employer's interest to persuade (or prod) them to work more hours. But of course even that's a gross simplification. There are lots of 'non-linear' considerations, e.g. burnout.