Because they need the money? Many people have little choice in the work they do. It's this shitty job or that shitty job or no job.
We had students who used their time to study, some read books, others used their beats for exercise, and some just plainly enjoyed a job that was stress free ninety nine percent and more of the time.
At first when I worked there, coding a wide range of programs including payroll, I was a bit distressed at the pay scales. However after meeting many different guards I came to realize that many saw advantages I didn't and they could not understand why I wanted to work so hard, all day and sometimes more at a computer.
Are you talking about short time scales, or periods of years?