Android in general is not doing it, but Samsung has implemented the feature for their devices.Almost every decision Apple makes for the iPhone and iPad is geared towards battery life because that's primarily what people ask for on a phone and tablet. Out of a thousand users probably 999 are asking for battery life and 1 is asking to play multiple media files at the same time. Music faces out during phone calls and video conferencing for the same reason.
If Android devices aren't optimizing like that it's not something they can boast about.
And it's not necessary about having Spotify and Apple Music playing songs together, that's silly. But some video playing in the background while audio comes from a different source. I think eventually with the introduction of Stage Manager we will see this kind of feature... I'm guessing Apple is trying to do their due diligence when it comes to adding it.
Because I like the new Control Center that's added... giving developers the ability to add actions to it. I suspect we could see a YT, Spotify dedicated volume switcher.