The Durham meat plant is a horrible, filthy, unsafe place, but people are able to stand working there because they are desperate for work. Unfortunately for the labor pool, there are far more unskilled immigrant workers looking for jobs than there are jobs to be had. This means that people will put up with almost any conditions just to have a job.
We learn, for example, that a worker who is a minute late for work is docked a whole hour's pay. People put up with this unfairness because they know they can be very easily replaced. As the narrator states:
While there are so many who are anxious to work as you wish, there is no occasion for incommoding yourself with those who must work otherwise.
In other words, people are so willing to do whatever the the boss wishes that the boss finds no reason to inconvenience himself in the least for the employees. Even Ona can't get a day off without pay the day after her wedding unless she wants to risk losing her job.
Sinclair depicts a time before workers had any rights or job protections: no minimum wage, no unemployment insurance, no workers' compensation, no paid time off. They put up with terrible positions like those at Durham's because they had little choice. Jurgis's father, against Jurgis's wishes, even pays a supervisor a third of his paycheck to get a job at the meat packing plant.