What I want to know: Why can't these phones use plastic screens? Good acrylic is more clear than glass, lighter, safer, more durable. I wouldn't be shocked if iPhones were designed to shatter easily. I'm seriously thinking of switching back to my ancient Blackberry: plastic screen that doesn't crack, battery lasts a week, make calls by pressing one button, I can turn it off and on easily...
The capacitive technology that powers multitouch screens requires (or works much better with) glass, I believe. Also, LCD technology itself requires glass. The less and thinner the layers, the closer it feels like your finger gets to the image, and the better the UX is. There are iPhone cases that essentially do what you are suggesting with a plastic overlay.
If there are glass layers in there somewhere, maybe they're somehow protected by the plastic. Or maybe the all-out antiscratch optimized glass in iDevices is just much more brittle than regular glass.