We could now - but do you know how much a diamond screen "glass" would cost? A sheet of diamond 5" long and a mil or two thick? Do you really want your car to cost less than your phone? (There's a problem, though - scratch-proof means hard, shatter-proof means soft [or non-crystalline, which is mostly opaque]. So until Mr. Scott really invents transparent aluminum that's as hard as diamond ...)
It's a lot cheaper to replace 3 for $5 screen protectors every year. As for shatterproof, use a TPU case with a lip and exercise just a little care. (The only phone I've ever "dropped" was a Nextel i35S, and I used to throw it against the wall to demonstrate what a rugged phone should be. [The battery cover would fly off, the battery would pop out, but I must have thrown the same phone a few hundred times as hard as I could - even down to concrete - and stopped using it when Nextel stopped using iDen. Driving a tank over it might have done some damage.]) But I treat normal cellpones like rotten eggs - I don't drop them.