Keeping Lazy Workers? Why?

Is there a reason management won't let go a new lazy worker? I'm talking about sleeping on the job, and most times just sit there while others do the work. Everyone complains, everyone. The managers know what he is not doing, yet he is still there. He even abandoned his shift one time and just left. Why at some places, managers are not quick to fire? It is just going to make the good workers leave.