No Stupid Questions

People die.

In cases where someone meets an unfortunate grizzly end, like being eaten, there's an obvious reason. But more often than not, people just stop being alive.

Imagine you have no knowledge of science, how would you explain this? An hour ago, this body could move, could breathe, could do normal things. Now it can't.

Something has changed. Something is missing. What was once a person is now a thing, a body.

It stands to reason that the missing bit is the key to what makes people human. It's clearly not a physical thing - the body looks the same - so it must be something intangible.

Tie this to the fact that people are very good at detecting other people around them. We're especially good at sensing when we're being watched (in person, not through cameras, obviously). We also find ourselves in situations where we feel like we're being watched when no one possibly can be watching.

So we have a fundamental element of human-ness as something intangible, and we also have situations where it feels like someone is there when there's no-one around.

It wouldn't take a massive leap to associate the two.

Once you have human spirits established as a fact, it's not such a stretch to imagine other intangible beings are responsible for other unexplainable elements of the world - the weather, crop yields, health, fertility, etc.

Your question is broader than you think.

For one culture to have an idea and share it to others isn't uncommon. Pyramids are in Egypt and Mesoamerica. Giants exist in many cultural myths (see https://en.wikipedia.org/wiki/List_of_giants_in_mythology_and_folklore) as are ghosts (https://en.wikipedia.org/wiki/List_of_ghosts).

You see, an idea that's communicated is a meme.

" A meme acts as a unit for carrying cultural ideas, symbols, or practices, that can be transmitted from one mind to another through writing, speech, gestures, rituals, or other imitable phenomena with a mimicked theme. "

https://en.wikipedia.org/wiki/Meme

If you're considering spirit as a soul

https://www.planksip.org/the-concept-of-the-soul-in-ancient-philosophy-and-soul-1763067454225/

TL;DR: So, why does it exist across cultures? It's a meme.

Because people want to believe that there is something special about them that distinguishes them from other animals, or from the merely physical.

And also, many people want to believe there is something in them that will survive past death.

yeah because its kinda a common thing to think of in the generic. souls, spirits, ghosts, faeries, etc. Sometimes it people sometimes it animals or land or plants or oceans or elements or whatnot. Basically most either said a god does a thing or the thing itself has internal invisible life to it. They are honestly not all that similar in detail.

People want to have a spiritual aspect to their lives, mythology just allows it, it is shared because people's minds are all similarly structured.