Stock headphones
Many people judge audio quality using the stock headphones which are pretty rubbish.
If you're bothered about sound quality then you'd not listen to MP3. Lossless AAC or FLAC would be better.
Advances in sound quality went out years ago anyway. People have generally decided they prefer convenience to quality. Being able to have gigs of audio on tap in a small device is much more desirable than lugging around a CD player and CDs.
Not to mention being able to purchase music on the device and have it download (on smartphones anyway).
Super audio CD and DVD Audio never took off, even though they would have offered 24-bit 96Khz playback (even 192Khz), which is on average what people record their music at.
Seems pretty crazy that blu-ray films have better audio quality now than dedicated music players.
Blu-ray can do 16, 20 or 24-bit range with 6 channels at 192KHz sample rate, 8 channel at 96KHz or 48KHz sample rate.
So even the lowest quality Blu-ray audio specification (16-bit 48KHz) is in excess of CD which is 16-bit 44.1KHz.