Just want to throw out there that "looking stable in the viewfinder" and "being stable" are not the same. There is so much post processing on the images (plural) taken on your iPhone to composite them into the final image they want to make sure there's a baseline to the quality going into the algorithm.
As far as not being available on the Pro, clearly they are having issues keeping up with demand for the Max phones alone. Wait a year and you'll almost certainly get it on the 16s. (Yeah, I know, waiting is unheard of...)