Competing services can apply to be included in the program based on technical specifications (limit your bitrate, clearly identify the traffic as video).
Unfortunately, as a consumer, you don't have the technical resources to integrate your video provider with T-Mobile for them to support BingeOn. Your options are to wait, or move to another cellular provider (which doesn't, of course, have BingeOn).
That is why BingeOn is offered. T-Mobile receives a network efficiency benefit, which they pass along to their users as unlimited video. Just because the user complains they want something, doesn't mean they get it.
Complexity also increases as each ISP gets to choose their own rules.
Besides, I, as a consumer want to be able to use Amazon Prime Music / Video at 0, whether Amazon wants to take the time to set it up or not.