While narration is often in simple present tense, especially post-World War II and contemporary narrative and that influenced by Hemingway and Steinbeck, it is not always in simple present tense. Some examples of different tenses are Fielding's Tom Jones and Dostoevsky's Crime and Punishment and Dickens' David Copperfield.
Fielding's Tome Jones:
This gentleman had in his youth married a very worthy and beautiful woman, of whom he had been extremely fond: by her he had three children, all of whom died in their infancy. He had likewise had the misfortune of burying this beloved wife herself ... he would often talk a little whimsically ... and considered his wife as only gone a little before him, a journey which he should most certainly, sooner or later, take after her; ...
Dostoevsky's Crime and Punishment:
But Raskolnikov had become superstitious of late. The traces of superstition remained in him long after, and were almost ineradicable. ... In the previous winter a student he knew called Pokorev, who had left for Harkov, had chanced in conversation to give him the address of Alyona Ivanovna, ....
Dickens' David Copperfield:
We walked about on the cliff after that, and sat on the grass, and looked at things through a telescope—I could make out nothing myself when it was put to my eye, but I pretended I could—and then we came back to the hotel to an early dinner.
Many texts do function in terms of present tense voice. One could assume that authors do this to bring readers into the story. Some may read texts written in passive voice as not relevant given it has already happened. Present voice allows readers to take an active role in the text- it tends to force readers to become part of the text (perhaps that is what the author is trying to do). Unfortunately, without knowing author intent, readers can only speculate as to why a particular text is written in present voice (active voice) instead of passive.
I disagree that the narration of literature always has to be in the present tense. In reality, the narration of a text is a result of a careful decision made by the author considering the impact that he or she wants to create. Thus some texts are definitely narrated using present tense, which gives a strong sense of immediacy and adds a vivid nature to the narration as we as readers join the narrator in what they are doing step by step. However, each narrative choice has both its positives and negatives, so you might want to analyse texts with other narrative styles to see the differences.
The active voice of a well-written present tense narration is a powerful tool in which the author keeps the reader engaged. Most readers want to feel as if they are part of the action, and present tense narration fills this need. Reading past tense narration produces a sense of inevitability in the reader--we know nothing can change as it has already happened. Present tense leaves us in suspense--knowing anything can still happen.
I don't know that it is always in the simple present tense, but as a teacher and student of writing, it is always much more interesting to be in present tense and active voice. If written any other way, the writing is wordier, more dense, and not as fun or easy to read. When you make the reader work too hard, he/she will often give up. So, present tense with the help of foreshadowing and flashbacks to fill in the gaps with past events and hints for future events makes the reading more interesting and attention-getting.