Since day one, iOS has never let you just add MP3s to the music library on the phone directly - you've always had to sync it through iTunes on a computer. This

Since day one, iOS has never let you just add MP3s to the music library on the phone directly - you've always had to sync it through iTunes on a computer. This made sense when the behavior was inherited from iPods and two-way sync was difficult, but then they never had any incentive to add the ability once people stopped connecting their phones to computers anymore and they could just force people to buy all their music from them instead.