Because chances are your device has Bluetooth version 5.2, as all newer phones have and standard has been out for almost a decade now. That version supports both aptX and LC3 codecs. Both of which are significantly higher quality than default inital SBC. In fact aptX has been around since 2009. In 2009, Senheizer released first ever Bluetooth headset with aptX support. Later in 2016 we got aptX HD.
Even Sony's proprietary LDAC codec has been added to Android in version 8 and was officially supported by all Android devices since then. That codec can push 32 bits/96 kHz, which is more than enough quality.
Of course with all digital protocols, like Bluetooth is, received data is the same as transmitted data. If your headset audio quality is lacking, that is on headset, not protocol itself since Bluetooth supports high quality audio for decades now. Only time when Bluetooth will revert back to old codec which was "reasonable quality" as they describe it is if your headset doesn't support better codecs.