I bet these employees who suffer such exploitation don't have loyalty to the company and they will leave the moment they find the greener pasture with the same treatment or better. However, I cannot deny that many companies thrive like these since they are just lucky to have more people looking for a job than the job availability. This mostly happen to casual employees at the lower level of the chain. But on the higher level job where education, mastery, management, and critical thinking are involved, companies are taking good care of them since they know that the services offered by these individuals are very essential to the company's success.
Definitely, if they see a better opportunity, they will gladly accept, but the problem too is that in some third-world countries where it is usually so difficult to secure a good job, citizens just manage whatever job they have been employed on, no matter the pay or unfair reward for their efforts, and like you said, if it's for a company where the required staff that is needed is less compared to the high number of applicants, those that would be employed would consider themselves lucky, and at such a time, they can decide to work with such company, no matter the condition.
There was this company that my one of my uncle was working with last year (a refinery), but the company usually delays paying salaries; sometimes they can even owe them for three months. Despite the debt the company was owing their staff, they still assured them that they would pay, but the day they were paid, it was only for two months that the company paid for and still owe for one month. Luckily, he got another job with a better offer and pay.