As meta almost-showed, the reason "why" is to keep those platforms from interacting with federated-services on their own terms - they can't control the interaction if no one uses their implimentation.

Leave it to them, and its embrace-extend-extinguish all the way down. Reddit and the rest have used AI-training as an excuse to lock-down and claim owner-ship of user-generated content. Letting them keep users from using our own content how we please would be a mistake.