I know other people who have it because they want better grip or they just want to customize the looks of their phone. Protection is not always the #1 reason.
Now i drop my phone once and the back is shattered. Honestly getting pretty sick of the direction apple has gone with the iphone. I just want the iphone 7 with better battery life and some hardware upgrades.
They probably looked at the numbers and realized 98% of people put cases on their phones anyway so the disadvantages of glass weren't too bad.
On the other hand, I routinely keep phones 5 years, and putting the case omit greatly increases the odds of it making it that long.
On the third(?) hand, cases themselves are kind of a crapshoot for durability. I cracked the corners of an Otterbox case, glued them, cracked them again, and finally switched to an aluminum Ghostek case that's holding up ok.