Yeah... this is one equation that I can't solve. I was at Disney recently, and the number of people that did not have their phones in a case was extremely low. And the ones that did either had the less expensive phones (lots of un-cased Moto Gs, for instance), or older phones that were already beat up (I lost count of how many shattered iPhone 5/5S screens I saw). Seeing a naked S6 or iPhone 6S+ was like seeing a unicorn.
So if nearly every customer slaps their phone in a case as soon as they take it out of the package, why bother? I mean, couldn't they save a lot of money and just make a durable chassis, one that replicates the protection of a case? Surely the buying public is savvy enough to recognize and appreciate this move.
Nope...
Those expensive and delicate chassis's are one of the primary selling point. Look at the current crop of 'flagships'. It's a sea of glass and aluminum. Had anyone eschewed those materials for a high quality and near bulletproof poly-carbonate skin, reviews would have howled about how cheap they felt and so on... People flocking into their carrier stores would probably walk right past the tough-as-nails phone and go to the shiny ones, buy them and then slap their Precious in a thick Otterbox tank-like shell, never to be seen again until it is time to replace it with another one.
But it is what it is.