Because they're morons and don't use a case, or a case with proper edge protection. Rarely if ever a phone will fall on a protruding object. And even then, there are glass screen protectors that help with that as well.
For phones that push $1000-1500+ in some configurations, its utterly moronic not use a case. And if you're filthy rich to do so, don't complain if they break.
I had an iPhone 3GS bounce off a subway train, and fall face flat on the subway tracks. After the transit employee retrieved it for me, both of us couldn't believe the screen wasn't broken. It had a case with a decent lip over the edges of the screen.
That could not have happened at College or Union station ... just couldn't have