Why are Aunts and Uncles used?

And don’t forget uncle Donald Duck.
I see your point, and I am curious too.

“Trenton Lee Stewart’s Mr. Benedict from The Mysterious Benedict Society explains this best:

“[I]t would be simply impossible for me to put at risk any child who wasn’t alone. No matter how important the cause, parents are disinclined to send their children into danger, as well they should be.”

“My favorite children’s novels always feature an adventurous kid as the main character. These types of books present a problem for their authors: how can a child realistically put themselves in danger with their parents waiting for them at home? Writers can add a secret identity or other devices to justify this dilemma, but otherwise a hero’s parents can create a lot of plot holes. Sometimes it’s just more convenient to avoid the mom and dad altogether.”
https://bookriot.com/many-childrens-book-characters-orphans/

Like the response above me, it’s a classic “Hero trope” that the main character has to be a broken person, which is the early stage of a transformation to later become super amazing, basically.

They take all their pain and suffering and use that as motivation to evolve and save the world.

@cheebdragon Interesting. But in the two cases presented, it doesn’t really hold, does it? Peter Parker adored his Aunt May and it was the loss of Uncle Ben that made him want to do good with his new found powers. And Dorothy Gale wanted nothing but to get back home to Auntie Em and Uncle Henry (and Zeke, Huck, and Hickory). These teens were actually willing to put themselves into harm’s way to get back to their caregivers, weren’t they? In both cases they viewed their guardians as their parents.

I think @cheebdragon is mostly correct. Parents are normal and therefore predictable.

Aunts and uncles introduce different story dynamics and story lines.

Think about the Andy Griffith show. There was Aunt Bea, but Andy wasn’t ever shown as married despite having a son Opie.

My Mom lost her mother during childbirth. Her Dad couldn’t deal, and split. My Mom was then passed around to aunts and uncles, sometimes changing addresses 9 times a year.

It adds an element of unknown. An element that this may not have happened had it been with Mom and Dad.
@elbanditoroso We know Opie had a Mom, but the show never depicted one. However, she must have left the family shortly after birth. That’s because in an episode of The Danny Thomas Show where Opie loses his pet turtle because it was stepped on, Andy reveals that he lost Opie’s mom when he was “the least little speck of a baby”