What you’re talking about isn’t a bad idea, I just think it has greater value. If you have 10,000 fans and only 100 of them subscribe to Apple Music, you’d be selling a product to only 1% of the people willing to pay for it. Trey Songz generates 50K a month from his Angel Network app where he basically sells birthday shoutouts, phone calls, etc. and a big part of that revenue generation is the fact that the app is free. Anyone can download it and use it. The money comes from in-app purchases. Apple Music will be exclusive to paying subscribers.
For Apple Music allowing artists to offer those extras would serve as incentive for people to use their platform over another. Charging for those extras would cancel out the benefit for them because fans wouldn’t see the value of paying $10 monthly plus however much for extras. Right now Spotify allows for artists to post “offers” on their profile pages where they can sell things like Face Time chats, etc.
As for artists, we're only making a penny per stream so why give anything away to incentivise more streaming when there’s maybe 20 times as much to be made selling it independently? The extras work on Tidal because the participating artists are co-owners and share in the overall revenue of the company. They’re not being paid per stream.