Sure, that works for certain contexts. But what about a game, where your vision is used to aim a weapon? The app has to know where you are looking in order to know where to shoot. If it's an online game, then the information cannot be isolated on the device since you're shooting at other players.
Now imagine the game has advertisements, or loot boxes, or something else that the game company wants to know if you've looked at. How does Apple prevent them from learning that, if they're able to know what they need to know for the game to function as a regular FPS?