Because too many employers are content to take as much productivity as they can get while paying the minimum. In a perfect world, employees can leave and find a job that properly values their contributions. In a perfect world, employers would then have a hard time finding people to replace the departed employees. But we don't live in a perfect world. Employees need to eat, need a place to live, etc., so they cannot simply walk away from their employment, particularly when the other employers aren't offering more. So they aren't leaving, and employers aren't having to offer a fairer wage to recruit new employees.

Some will argue that the employee then needs to learn new skills to make themselves more valuable. Maybe that works for an individual, but it doesn't solve the overall problem. There are positions that require low-skills workers. The people that fill those positions have the same basic needs as high skill workers. If they don't get paid enough to survive, then nobody can afford to do those jobs. It might not take a great deal of skill to collect trash, but we would be living in filth if we only offered $4/hr to trash collectors.