Because it hasn't been announced. And I highly doubt it makes it in this beta cycle... they might make some tweaks to Stage Manager. But adding a feature where apps can stream simultaneously is something they would have mentioned in the preview page of iPadOS 17 or in the keynote beforehand.
I think soon we will have that feature... given users can have multiple apps onscreen with Stage Manager, it's one of those aspects where its evidently missing. It was rumored to be announced for iPadOS 17, but it didn't make the list.
And I'd imagine having a feature where users can manage volumes of multiple apps would be somewhere in the Control Center (which was rumored to be redesigned).
With how Apple basically unified the macOS menu bar and iOS/iPadOS status bar along with control center, I suspect that the Now Playing in iPad’s control center will function like Now Playing on Mac, where all apps that output music will be listed.
That’s exactly what I always thought. There’s no away to implement that in the current setup because that would be a mess - audio streams playing everywhere and no way to quickly control it.
We need a redesign on multiple levels or it may be restricted to stage manager. IDK.
Computers have been doing fine without it, macOS only recently added a way to manage all that with macOS Big Sur.
One thing Apple can do to indicate that an app is playing audio is what they already do in Safari for Mac, which is showing a sound glyph for each tab, so they can do that same for apps in the app switcher.
Likewise, implement the same Now Playing they do in macOS (which shows all apps that is outputting media).
Currently, apps can already overlap unless it’s outputting media.
Frustratingly, many apps unintentionally take this over and it becomes annoying.
You can’t even have multiple audio playing when you have multiple windows in stage manager. Truly baffling.