In addition to incentives, workplaces harness, among other things:

1. Reciprocity, and commitment/consistency effects. You accepted the job; they did their part by giving you money; you feel now like you have to work. (As an example of the power here: I recall a study in which individuals who were sent a survey, with some money inside as a "thank you for filling this survey out", had a much higher survey-completion rate than people who were credibly promised that those who returned the surveys would receive a larger sum of money after their surveys were received. Even though the first group had no incentive-structure (since they already had the money), only reciprocity.)

2. Work-conducive contexts. Workplaces try to set up a context in which the pieces needed to work are all available, distractions are relatively unavailable, and working is the expected activity for a certain set-aside portion of one's week.