It's not just codecs that are different between HFP and A2DP (although that is a big part of the issue). HFP uses an older way (SCO) of transferring the audio which gives much less bandwidth.

What we need is a new profile that uses A2DP in once direction and in the other for the microphone. I suspect the reason it has never happened is that there's a good chance of causing issues with existing devices that do at least work with HFP.