Only an Apple engineer could answer this fully but here are some of my ideas as to why this hasn't been done yet:
-They still support playback on devices with no TEE. Kinda defeats the point of implementing it until this is the case.
- They are wary of moving more functionality into their TEE as it increases the attack surface.
- If the platform is already "attested" and locked down as it is the case today, moving the playback to the TEE provides only a little bit of extra security.
- They are banking on the ARM Realm Management Extension[0] coming to their chips. This would be more of a "catch all" solution to fuck over the owners of their machines in new and exciting ways.
[0]: https://fuse.wikichip.org/news/5699/arm-introduces-its-confi...