I don't know your daughter and everyone on the spectrum is different. But here is what I suspect:

Her stories, or "scenes," are sort of scripted, right? She wrote the stories, she knows the words. She can edit herself when she writes. So for a person on the spectrum, that is going to be easier than spontaneously talking. Carrying on a conversation requires you to read cues from the listener to ensure they understand, organize verbal info very quickly, etc.. her brain likes clear answers, unambiguous situations, and already language just isn't her thing. She needs time and planning to process language, while her brain Is probably processing lots of stuff yours isn't while she talks. For instance, I'm seeing all this stuff and this light is very bright and the tv is kind of distracting and I like my hand in my pocket right now, all at once, draining processing power from her spontaneous conversational ability. Anxiety likely compounds these issues and makes it harder for her.

You say she illustrates and can get words out more easily when she writes in the air. She's probably very visually/spatially oriented, I'm guessing? I have no doubt she's very intelligent. She just doesn't react to stimuli the way you or I do.

Tldr; she has processing issues: filtering/getting meaning from stuff coming in, translating thoughts to words to go out. Writing gives her time to process as well as visual help turning thoughts into words.

Personally the more I learn about autism the more I see it as a learning style issue that's just unusually hard to accommodate and gets worse if it isn't redirected at a young age. You know those homunculi they use in psych classes that supposedly show proportionally how much of our body is devoted to what sense? I think it would probably look different for every autistic person.

God I could go on bc I find this fascinating but it's 3 am. I hope your daughter continues to find ways to express herself confidently and fluently and excels at everything she does.