Sharing, and believing in, fictions is how distilled units of information are efficiently passed down generations and is one of the bedrocks of our development as a species. This is what allows us to have laws and corporations and agreeing to drive on particular side of the road.

Yuval Noah Harari covers this more eloquently in his book Sapiens. You would definitely dig the relevant chapters.

Humans have brains and are social creatures. Brains love processing information. Learn from things that happen to people around you or even fictional ones. Are curious and want to connect information. Also we’re built to empathize with others to be able to form groups/a society. We will even empathize with fictional characters and inanimate objects that have big eyes and a mouth.

Evolution gave that to us. We wouldn’t have survived as a species if we had cared zero what happened to our neighbours. A story about Harry Potter is probably on the same level for our brains.

The world is complicated and difficult to navigate. Stories usually give you a simplified world where it’s easier to understand and relate to. Just think of most religious stories and myths, they exist to explain something unexplainable (how the world was created) or how to behave in a society (cautionary tales and parabolas)

We tell ourselves stories about our own actions, choices and beliefs (internally). We desperately want to believe that our own behavior is consistent and coherent. Fitting our behavior into a narrative helps us maintain this illusion (“I did Y thing because X thing happened to me, and then Z thing happened because I did Y”).

We tend to assume that cause-and-effect relationships are true and real, that we perceive causes and effects correctly, that we associate causes with effects accurately, and that we perceive all causes and effects that are relevant. These assumptions give us a narrative structure through which we make sense of our own behavior… though they are about as reliable as any of our other assumptions.

Anyway, the upshot is that framing the actions, choices and beliefs of others into a story helps us to understand and empathize with them (or condemn and villify them). The stories are a construct formed mostly from confirmation bias, but we struggle to make sense of reality without them.

This is an interesting question, I hope someone answers!

Edit: I did some googling npr

On functional MRI scans, many different areas of the brain light up when someone is listening to a narrative, Neeley says — not only the networks involved in language processing, but other neural circuits, too. One study of listeners found that the brain networks that process emotions arising from sounds — along with areas involved in movement — were activated, especially during the emotional parts of the story.

Also interesting, maybe not as related to the question:

As you hear a story unfold, your brain waves actually start to synchronize with those of the storyteller, says Uri Hasson, professor of psychology and neuroscience at Princeton University. When he and his research team recorded the brain activity in two people as one person told a story and the other listened, they found that the greater the listener’s comprehension, the more closely the brain wave patterns mirrored those of the storyteller.

I would say the simple answer is in why we still teach “story problems” in math.

The “story” of the math problem gives us a way to contextualize knowledge in how it can be applied to real life.

Most stories impart social knowledge, not math knowledge. Stories are primarily about relationships and how to (or how not to) navigate them.

In philosophy, what you’re asking about is called the paradox of fiction.