Sapphire on a watch makes sense because it means your watch resists scratches for years and years. My Omega (with George Daniels' glorious co-axial escapement, watch nerds!) has a sapphire crystal; despite being sorely ill-treated it is pristine, some six years later.

But the half-life of a phone is what? Two years tops? (Yes, I know there are people who keep the same phone forever; they're not most people)

Also, sapphire's hardness comes at a price: brittleness. There's a reason that NASA spaceflight-qualified watches predominantly have Perspex or plexiglass crystals. (Or are Casio G-Shocks. Quel surprise!)

Seems like a bit of checkbox engineering to me.