[irae]

All the sibling comments are better guesses, but I would also guess there could be security implications on exposing lower level access. Having it all proprietary and undocumented is itself a way of making it harder to exploit. Albeit, as mentioned, not having to settle ABI is way more likely the primary reason.

[kmeisthax]

Apple Silicon has IOMMUs on everything - you generally can't exploit a bug in a coprocessor to gain more access on the main application processor (or another coprocessor). The only hardware bugs with security implications we've found was stuff like M1RACLES, which is merely a covert channel (and it's discoverer doesn't even think it's a problem). Apple does a pretty good job of making sure even their private/internal stuff is secure.