Metal cases give way, too— permanently. When they give way, they dent, shrinking the space available for the glass screen and, inevitably, cracking it.
Glass cases/backs obviously easily crack themselves.
Making phone housings out of something other than plastic is an obviously stupid kind of fetishism for certain materials as hallmarks of a vague 'quality' regardless of context.
Idk about the screens. Do we need glass for touchscreens as we know them to work really well? Can they be brighter or something? Or are they largely unnecessary, too?