Answer:
Everybody is different and it responds to stress conditions in different ways. This is the reason the people progress in different ways.
The amount of time that is required for recovery is different in different people. Keeping this condition in mind, exercises are designed individually and specifically.
Exercising for a specific part makes that part more stronger and person happier by seeing the areas he is working on.
hence, the exercises are made specific to encourage in the area of interest in the body.