Because the bandwidth, bitrate and audio codec for HFP profile hasn't been upgraded in a decade. AptX and co. are only used in stereo profile with no mic suppor

Because the bandwidth, bitrate and audio codec for HFP profile hasn't been upgraded in a decade. AptX and co. are only used in stereo profile with no mic support. As soon as you request bidirectional audio it downgrades to horrible old codecs.