Earlier quoted context omitted.
Quite simply because longer-life phones would get trashed even more for not having the features they traded to get longer battery life. Imagine what the reviews would be if the next iPhone is twice as thick and double the weight. That it goes 3 days without a charge for most people would be a footnote to an otherwise poor review.
The irony of course is that after the Apple engineering team spends months trimming off that extra millimeter of thickness, most of their users still go out and get an otterbox that triples the thickness. Of course, even if they designed an iPhone only twice as thick with triple the battery life and no need for a case, it would still be panned.
Anecdotal (as an iOS dev), yes, but how hard would it be to add back a fraction of a mm in casing above the screen to protect from falls?