Substitute teaching can be a thankless job...especially if the "sub" isn't very good at it. "Subbing" is one of the areas that sets education apart from other professions because they are required to be there when a teacher is out sick. They're there to follow the plans left by the teacher and make sure students continue their learning. Unfortunately, not all of them are qualified t be in the classroom.
Some districts belong to a consortium that provides substitutes from a list of certified teachers. Other schools may have a person on staff who fills the requests. No matter which service is used, there seem to be substitutes who do not care about making a good impression in the building.