Here's why:
* Autobiography: A person's account of their own life. Since it's the person's own story, they would use "I" and "me" to tell it.
* Third person: A narrative perspective using pronouns like "he," "she," "it," and "they."
Think of it this way:
* Autobiography: "I was born in a small town..."
* Third person: "He was born in a small town..."