Because your boss expects you to work for no pay

You’re on a salary: your employment contract makes it quite clear how many hours you’re expected to work and how much you should be paid for those hours.

Or you’re on a union-negotiated hourly wage: the collective agreement makes it quite clear that you should be paid that wage for every hour your work.

Or you’re on minimum wage: the law makes it quite clear that you should not be paid any less than this.

Your boss regularly expects you to work additional hours for no additional pay.

If you complain, your boss responds furiously.

It’s quite clear that your boss has nothing but contempt your employment contract, or the collective agreement, or the minimum wage.

You can’t cope with the fury or the implicit threat of being fired, so you go on working for no pay.