Remember to not generalize and assume that "all" authors write in third person. Autobiographies and memoirs count too and they are written in first person. However, third person omniscient is the most popular point of view because it allows the author to give an objective and impartial perspective to the story. This avoids biased writing, or partiality towards a certain topic, or character.
Another reason why authors feel comfortable with a third person point of view for their narrative is because this point of view allots the author to treat more than one character with equal attention and focus. For example, although Jane Austen often tells her stories from the point of view of the heroine, her use of a third person point of view allows her to also focus on the lives of the other female character. This, in turn, makes for a wonderful opportunity to compare and contrast characters. After all, we get just as much information from one or the other.
Using Austen to illustrate this latter statement, just imagine what would happen if Elizabeth were to tell the story of the Bennet sisters from her (very) prejudiced point of view. Regardless of whether she is the heroine, or the main character, we would miss so much! She could have not known about Darcy's true feelings. What about Wickham's lies and wickedness? Those were facts that she only thought she knew about. As a narrator, she would have been quite limited.
This leads to the last point: Third person subjective narratives are limitless. The stories are told from a wider scope and from a more universal perspective: Anything can happen to anybody. The connection that is made with the reader is more effective this way. It is simply a way to tell a story and make it as real as possible.