Why are you so against a magic origin story?
A number of reasons. I think that explaining her magic will just make it less “cool” (pun unintentional). Part of what makes her powers cool (and just magic in general) is not understanding them. Well, we can understand how they work (slowing down molecular movement), but knowing where they came from would be grounding them in a way as to make them somewhat boring. Not knowing why she has them is part of what makes her interesting. And explaining it takes that away.
Additionally, any explanation they give cannot possibly be better than any of the ones already theorized in the fandom. We have so many of them; is she descended from an ancient sorcerer? was she cursed by an evil witch? maybe it’s part of a prophecy? what if she was granted this power by the gods? what if she IS a god? Anything that Disney gives us will inevitably be disappointing and less interesting than the myriad of ideas we already have.
Also, having the entire story centered around that just doesn’t sound that interesting. There are far more interesting stories that could be told, and hopefully will be told.
Does that make sense?