Because stories are important.
Our brain loves stories because they always carries with emotion, as well as focus on the record in memory the stories help us remember the words to be able to recreate these stories with similar words.
The human memory is not a database. The memory does not have this function (purpose). Many people believe that we have pure memories of the past, but we do not. How many thoughts produce yesterday? Thousands! We can remember all the thoughts, with the exact string of words? Certainly not, but if we remember people, environments and circumstances with which we interact, rebuild of the thoughts we had yesterday. So the stories are important they contain people, environment and circumstances that help you remember words.
We always reconstruct the past based on experience. So we conclude that the memory function is the basis for creating our representation of the past and not for bringing events in full.