Sapphire glass has also been used in premium wrist watches for ages. These watches don't scratch. And when you drop the watch on the face from table height, say, it's more likely that the mechanics inside break than the glass.
I suppose that it is more difficult to handle in production, especially when you're aiming at a large surface area. But if scanners have it, this issue does not seem to be insurmountable.
Many customers would probably pay extra if they would get a phone that's as tough as a wrist watch but does not require a case.
Unbreakable is an exaggeration. I want something that will not break when dropped from 4 to 5 feet onto irregualar concrete. Phone are pretty good, but mine is glass front and back and I dont wrap it in a case. Have dropped it a few times and got lucky. I just dont want to rely on that luck.
Having it slide across an otherwise flat surface with indeterminate grit on it usually does the trick.
Aluminium oxide, for example - it comes off as white powder on motorcycle engines - and is pretty hard stuff, the same stuff as sapphire and ruby. That can end up on surfaces via motorcycle gloves if you ride motorbikes, if not from your own bike, then grit thrown up by tyres on a wet day.
I mean, phones don't scratch easily, but they do acquire a tiny scratch or two every few months, and a bad scratch maybe once or twice a year.
Apple tried to use sapphire glass for phone screens around 2014, but only succeeded in driving their sapphire maker into bankruptcy.