I bought a cheap ‘mp3 player’ from eBay when I was trying to use my phone less, but it was rubbish: no library function (just the files, listed in the order they were copied), m4a support was limited to low-complexity), Bluetooth dropped if I moved my head.
I was surprised, because there’s definitely system-on-a-chip + open-source software combinations that would make a good, cheap player. The kind of places that make them though, will always prefer to use even-cheaper components than what the final retail price would suggest.
So, because everyone uses their phone for music listening now, it’s hard to get a dumb player. They’re either rubbish, like I bought, or they’re completely at the other end of the scale (as in more expensive than a phone)