It's a quirk of the Qualcomm chipset they used in the Quest. It has an extra DSP accelerator that was originally unused, but they managed to repurpose it for hand tracking. It was an opportunistic play, as I understand it.
It feels like they're trying to exclude anyone who wants to play non-Oculus-published games.
It's in here somewhere. It's also extensively covered in the hour breakout session by the folks who implemented it, which is uploaded somewhere though I can't find the link atm.