The issue is that a lot of the use cases for the processes you mention have soft real time requirements where GC pauses are unacceptable, and the underlying OS'

The issue is that a lot of the use cases for the processes you mention have soft real time requirements where GC pauses are unacceptable, and the underlying OS's have differing world views when it comes to dealing with audio (even on the same OS, there may be different driver APIs with fundamentally different approaches to audio I/O!). So having a simple audio API + ability to do pro-audio like things in JS is not really possible in a browser. They have to expose a complex framework with a nuanced API to the underlying engine to make some of this stuff even possible in a web browser.