Why do authors put archetypes in stories?
No story is original. All stories are derived from familiar themes and characters that resonate with the human situation. Hence, the archetype.
Archetypes are like the "mold" of a character. Some common archetypes are: "The Knight", "The Mentor", "The Chosen One", "The Princess", "The Mother", "The Lover", "The Sidekick". Authors put archetypes because these characters are the ones people seem to "know" immediately.
Superman, Merlin, Harry Potter, Buttercup, Mammy, Juliet, and Donald Duck are only a few examples of the archetypes I have written above respectively. Archetypes transcend ages and generally give the first "sketch" of the character. The general sketch, not the specifics.
A good author can use many archetypes and come up with a fresh story. At the end of the day, crafting each character's "own" beliefs, ambitions, dreams, and passions cover the archetype and subsequently give birth to a fictional individual.
Archetypes aren't inherently bad. Stories need archetypes. However, they must be used cautiously and creatively. A character should not be left boxed in his archetype for a whole novel's worth.