Why is the back of the iphone made of glass in the first place. At least before it was like a usb key... you know 50/50 chance of breaking. Now they're gone full lightning cable and made it a 100% chance that some glass would break.
The kind of people who, when their $1000 phone breaks, will simply go to the Apple store and buy another.
So think of it this way: If having a fully glass front increases sales of replacement parts and devices, then having both the front AND back glass will DOUBLE those repeat sales.
Apple is not a nonprofit, and they didn't become one of the wealthiest corporations on the planet by being generous and altruistic.