Does MacOS even have OS level independent volume control for each app yet?
I've still never cared and don't know why I should. What am I missing? I've used systems with that, but never bothered to try using it because... why would I? Every media player has a volume control, YouTube has a volume control, every game has a volume control (usually multiple, for different types of sound), and 99% of the time I just want everything at about the same level anyway. The last thing I want to do is have another place a given program could have its volume set or muted or whatever, so then I have another place to look when it's not doing what I want. One per-program (built in to the programs) and one global is quite enough.
I'm not too sure how this is handled on other platforms, but pre-PA the in-app volume control would often modify the global mixer, rather than implement an in-app mixer/attenuator, which is definitely not what you usually want. In most Linux apps these days, those controls manipulate and are synchronized with the PA mixer, so there's still only one actual mixer, they are the same control. I assume that other OSes also do per-stream mixing in a similar manner, and just choose not to expose the mixer for whatever reason.
As much as people complain about PulseAudio it's always worked well for me, and from what I've seen of Windows and macOS alternatives, it's more featureful and usable out of the box.
Why do we need `open()` anyway?
(this is sarcasm, the OS exists only to solve app problems).