Bluetooth audio was originally aimed at handsfree headsets and speakerphones, and used a high-compression codec called SBC. When A2DP was invented SBC was mandatory, but better codecs could be used - if both ends supported them. For licensing reasons better codecs are often not included in either the source or sink. And, of course, Apple only talks to Apple.

The reason I know all this is that I send iPlayer audio from my Android tablet to my hifi via a raspberry pi - and the quality is awful (though better than the Android speakers) with frequent stuttering. The reason is SBC. It sounds much better with a cable.

But with my son's Sony Bluetooth headphones it's excellent.