Standard pretty ancient fairytales (which are quite horrific!) prettified & transferred to the cinema by Disney.
200 years ago or earlier being orphaned or abandoned was quite common........poverty, cholera & other diseases (much shorter lifespans), wars & etc. made the lives of the lower-class especially literally Hell, the children's even more so.

I just figured it was a simpler to write a story with only one parent to interact with. Basic laziness!

Something I have noticed about Disney films is that they perpetuate stereotypes and I really get hot under the collar in the way they portray female roles when their main audience is young children.

I recommend watching Jordon Peterson's YouTube lectures on Pinocchio. He explains the psychology behind the story. Here's a relatively short one:

Omg Disney movies! I didn’t let my kids watch any when they were young because they are horrible examples. Snow White and sleeping beauty powerless and basically dead= nothing without a man. Beauty and the beast- are you f-ing kidding me Stockholm syndrome! Ariel makes a deal with the devil and runs away from home at 16 chasing a boy she hasn’t even met!!
Am I the only who sees the stories this way?

Superheroes too?

Plead ignorance

Yeah..and kids didn't live long either in past centuries in England. Parents didn't name their kids until they were two years old, because most children died before then.

It's also in just about every fairy tail. One of my college teachers explained that fairy tales were originally a way for kids to address the common fear of losing a parent, a common thing indeed in the times when the stories came to be. Then the character has a really rough time but eventually comes out OK. So not so much about the loss of innocence, but to encourage & strengthen a child.