Mostly power. Phones just don't offer enough to drive demanding headphones or earphones.

There are dongles to address this issue, but they can drain the battery pretty fast. I don't want to drain my phone battery just because I'm listening to music.

If you have an iOS device, having offline music has plenty of limitations. It takes a ton of space that I'd rather save for pictures, videos and apps. And you have to sync your phone with a MacBook which is a hassle if you have more than one.