Because there are also other people that are more desperate, and are willing to work more for less.
There is a finite supply of labor, and we are not anywhere near a level of technology where we no longer have an unbounded demand for labor.
Well, isn't it exactly what's happening? As far as I can tell, in the unskilled job market, like retail or fast food, wages are what they are because anything less is either illegal or makes it better for an employee to stay on welfare.