It's because of fingerprinting. See https://webkit.org/tracking-prevention/#anti-fingerprinting for the official standards position.
I feel like they’re just using fingerprinting as an excuse to not implement functionality that people want. Of course, I don’t really understand the problem space, so it’s likely I’m missing something.
Do you actually believe this? Do you not default to more practical explinations like, maybe they don't consider it worthwhile to support because of engineering cost vs people that actually use it?
For example, I'd say 9/10 people I know who aren't tech literate have no idea the Health app exists on their iPhone. This includes people with Apple watches. Similarly it should be obvious that basically nobody ever knew about or used that one feature you liked.
When I said "functionality that people want", I didn't mean to imply that there was a critical mass of people that made MIDI support in a browser obviously worthwhile, I just meant that some people want it and they're being told it won't happen because of fingerprinting.
Nobody wants a web browser that "chooses" not to work on some % of websites. Users choose, browsers implement. They are welcome to gate this feature behind a per-site permission prompt if they think it's insecure.
i don't understand how this is a fingerprinting risk either, and i'm pretty sure i'm not missing anything.
...Well, I guess I was about to answer my own question there. Never change, Apple. Never change.