AVRCP is the control protocol for BT media (e.g. when you volume up or play/pause from your headset). I’m assuming you meant A2DP which is the protocol for receiving audio. HFP is totally separate from A2DP in that it’s a bidirectional audio stream. That bidirectionality is the difficult part - you have in theory tight time bounds for transmitting the data for latency reasons + these cheapo headsets have 1 radio which means they’re nominally limited to tx or rx. Since the assumption for the design is voice calls, you severely clip the microphone audio into the frequency band for speech & transmit mono which is why the audio quality is so bad (+ you further lossy encode it but the frequency reduction is probably the biggest dip). All of this has to fit on top of the PHY packet transport that time-divides access to the channel + has to coex with other 2.4 GHZ radios like WiFi (at least historically).

Now, could these designs be revisited? Probably. Maybe radios are small and cheap enough that we should have 2 and a protocol for bidirectional audio that allows independent streams over separate bands with software synchronizing and mixing audio. It’s also possible the audio encoders/decoders could be modernized with more modern compression techniques to improve audio quality without requiring changes to the bandwidth allocation. Bluetooth those is very ossified and the standards bodies move like molasses - most of the investment is in WiFi and radio vendors are more interested in milking what exists now (or coming up with proprietary solutions like aptx) than actually solving the problems.