Bluetooth is low bitrate. The audio codecs need to use a lot of compression. Old audio equipment are analog which is better because it doesn't have so much digital conversions to completely wreck sound.

Bluetooth is still reliant on its original SBC codec from the early 2000s or something. 20 year old tech. Due to this nobody really took BT audio adoption seriously until the past several years when the zeitgeist finally tipped. Suddenly wireless headphones were every where.

I think maybe it was when Apple got rid of headphone jack. So the rest of the industry caved. And we all just handwave away how bluetooth audio has always sucked.

For compatibility every device maker sticks to that 20 year old common denominator. There are proprietary codecs that are supposed be better quality but then you get all the joys of cross compatibility hell. If your devices aren't inter-compatible they'll fall back to the common denominator. The basic SBC codec. Even with better quality codec they can only do so much with limited wireless bitrate.

Fun fact. There is higher quality configuration for the SBC codec but nobody configures it in software when making their device. People say it's indistinguishable from the highest quality proprietary codecs. But audio can subjective so eh...

Even if you were to enable the better configuration for SBC. All the devices out there in the world are built with the default configuration. No two devices sender/receiver will ever both use the better config. So it's impossible to fix this.

It doesn't matter anymore since all this in the process of being superseded by Bluethooth 5 audio. Which throws away all that and tries to do it all over again. It's still reliant on low bitrate wireless protocol though. So they can use whatever algorithmic trickery so they can claim produce perceptually indistinguishable from CD quality or lossless quality or whatever.

I'm sure there will always be people that say they can tell the difference. I don't doubt people can because it's simply not the same audio but a disassembly into bits for wireless transmission. Then reconstituted on the other-side as near as possible to the original.