I have the headcanon that the sky kids are homunculus that have fallen stars as their powersource made during the ancient civilization. Unknown of their purpose but to explore and learn from what was once was by said mimicry in hopes of finding their original purpose. With the stars, they can truly never "die" or at least have lifespans that will exceed written time. They have no true context but to observe.
The thing about the setting of Sky COTL is that even though the player characters can all fly the world is built by an ancient civilization of people who could not and pretty much all the NPCs you will interact with outside of the dev inserts during anniversaries etc. are spirits of those people who could not fly. as a sky kid you are functionally just a mimic that takes on the appearance of the ancestors and picks up their gestures and fashions through imitation of their cultures you are exposed to. you don’t sneeze innately - you learn how to do a sneeze gesture by watching a spirit sneeze. this is how you learn how to wave to people, how to dance, how to applaud. when sky kids (players) communicate with each other, this is the language they have learned to do so, but because of how players communicate with each other, they are abstracting the language from the ancients into a sky kid culture of their own. player communities use the Bow emote frequently with each other in gratitude at every little interaction, which makes sky kids out to be funny little courteous guys. i got so offroaded from what i was going to say originally, which is that the pleasure of a world where you can fly is not necessarily one absent stairs, but one where stairs exist but where you can circumvent them with flight