I suspect the GP meant for voice. It’s just atrocious no matter what for any device, due to the HFP restriction.
The real problem comes with the HFP profile. Lack of decent codecs, and bandwidth limitations of Bluetooth make it really hard to solve. Not to mention the vast majority of devices you own can't upgrade codecs, or just won't because there aren't incentives.
There's always the risk of unknown patent claims, but the same is true for any other codec not older than 15 or so years.
I can only suspect why it's still so rare, but my guess would be either a fear of unknown patent claims, a lack of readily available hardware implementations, or a combination of both.