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.
I have the pixel buds A-series, and using my pixel 7a I can feel the quality going down when on a call. It might be just the latest buds, or my pixel 7a not supporting the codec, or the HFP not being set to use it ¯\_(ツ)_/¯