I think they were overly commited to that afterthought-idea and couldn’t make it work in time. They did a lot of work doing a 3-angled led surface instead of a simple screen and just shipped what they’ve got at the time. That’s probably why it’s complicated to put there just any image not tailored to it without even more weirdness, artifacts - and Apple can’t allow anything they haven’t done themselves to hurt their public image. And from the photoes of it’s insides, it looks like without that thing these googles could’ve been at least $500 cheaper.