Why doesn’t Apple warn people that their backup doesn’t include any music not purchased on iTunes?

I was sent a replacement phone with an out of date IOS, when doing the phone to phone transfer it got stuck for over 12 hours downloading and installing the IOS update on the replacement phone. I was instructed to do the IOS update manually and restore my phone from my last backup on the other phone. When restoring from backup I learned that iTunes backup only stores music purchased through Apple and now I am out 3,500+ songs. Please warn people about this.