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.
That's because the reviewers - and users - have been conditioned to think that thinner and lighter is the ultimate feature, even if it means sacrificing a ton of other things. The less material in a device the less it costs to make, and the more fragile it tends to be, meaning more profit for the producer. It's all about the marketing - convincing the consumers in the direction the companies want, so they can extract more money from them.
For a good example of this look at the new "portless wonder" Macbook and all the controversy around it...
With smartphones, the fact that I've seen many people put theirs in huge thick cases - and some of those cases have batteries too - indicates that we've probably passed the point of optimal size already. Personally I find it hard to pick one up or hold it comfortably by the edges if it's below ~10mm in thickness.
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.
That's not really irony, is it? A thinner phone in an Otterbox is still thinner than a thicker phone in an Otterbox. Also, there are plenty of people (myself included) who do not use cases, and instead prevent their phones from being damaged by not dropping them. I suspect that "most users" do not use a case anywhere near the thickness of an Otterbox.
Also, the lower weight makes the phone more resilient to drops. Anecdotal evidence: I've never seen a cracked iPhone 5, but cracked iPhone 4's were common.
Less volume means less mass, which means less kinetic energy that must be absorbed by the frame when dropped on the floor.
Thin and light phones don't need a protective case like thick and heavy phones do.
Interesting. I remember people complaining that the iPhone 6 was slippery, and of course it is heavier than the 5/5s. Do you also have numbers for 4/4s and 6 vs. 6 plus?
We don't have a lot of 6+. We may deploy more later as iPad Minis age out. FWIW, I dropped mine from a belt clip to the pavement and it shattered -- my first dead iPhone ever, and I've dropped my phones dozens of times from the 3gs up.
5/5s was better than 4/4s, but not dramatically so.
The irony being people then have to shove them in massive cases, often with built-in extra batteries, because the thin phones are vulnerable and have crap 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.