Legal/licensing issues aside, I can think of two ways in which this isn't necessarily the best comparison:

- without looking up the history/timeline my feeling is that personal music devices have been a thing for much longer than it's been possible to download Prime/Netflix etc. to a device. So maybe they just haven't caught up to that yet. But in addition...

- the audio will be roughly the same listening to music from your own device as from the IFE since you'll be using the same headphones, whereas if you watch a movie on IFE you will probably be watching it on a larger screen than on your personal device (depending on age of plane) and/or at a better viewing angle (eye-level vs. on the tray table)

unless you go for this approach I guess: https://www.instructables.com/id/Unu...s-Ziploc-Bags/

(or other airlines w/o TV screens that have put a slot in the seatback that can be used as a tablet stand)