Bluetooth isn't capable of higher quality audio transfer. Codecs by design remove portions of sound data with their compression and decompression. That's literally why they exist. You lose some quality. They just try to remove portions of audio you don't usually hear, but it is noticeable with high quality systems.

It's why your typical compressed digital song you get a hold of is around 2 1/2 MB. That's its hacked down form. You can get "lossless" songs at full quality, but the sizes will be closer to 30 MB. Bluetooth and the DAC's together can't handle that. If you want high quality, you can't use bluetooth.

The "digital" signal your talking about with Bluetooth is just that. A digital one, and that digital one is a conversion from the dac that dumbs the audio down into what can be sent and processed back in a timely manner.

Going from your phones dac straight to an analog system to wired headphones cuts the conversions needed in half. Instead of going from the phones dac to Bluetooth to the wireless headsets dac to the speakers, you just go from phones dac straight to the speakers. Your phones has a lot more space than those tiny earbuds, so phones will generally have a much better dac than the bt earbuds will. That's on top of the transfer limitations of Bluetooth.

Short and simplified version: your phone can have a better dac than bt earbuds and wired headphones allow you to skip over half the conversion and transfer process.