Shouldn't this type of password be stored in the devices keychain, which can be unlocked with the device's auth mechanism, and provides authentication services

Shouldn't this type of password be stored in the devices keychain, which can be unlocked with the device's auth mechanism, and provides authentication services without exposing the itunes password to app developers? That is, authenticate user on device -> verify recipient of auth info -> send auth message. I must be missing something since no-one seems to be suggesting this.