Limitations on Apple Watch apps?

I'm trying to figure out why there doesn't seem to be any video doorbells that support live video on the Apple Watch. Given the Camera app on the Apple Watch, it's obviously technically possible, and the "user experience" is no worse than it is with the provided Camera app (which provides a live camera view on the watch). I'd be worried that it couldn't do that *and* do two-way audio at the same time, but the realtime video was available at least four watch generations ago, and the latest watches have much more than 4x the processing power of older watches that provided smooth realtime video mirroring from the phone camera. Audio is obviously much less resource-intensive than video, so 2x should be more than enough to do this. Is there some Apple-imposed restriction on this type of app, or is it just that nobody has bothered to do it?