Earlier quoted context omitted.
That's easy... make the batteries easily replaceable... oh wait.
That would makes the phones considerably thicker and/or make the battery life significantly worse. I don't think replaceable batteries are really worth the tradeoffs given the constraints of a modern smartphone. Lithium batteries can be quite dangerous, so they need to be in a case to prevent damage that could cause them to catch fire. Then, the phone itself needs a battery door which adds even more thickness and pot…
Would it? The Lumia 950 has a user replaceable battery:
Thickness - Lumia 950: 8.2mm; iPhone XR: 8.3mm
Battery - Lumia 950: 3000mAh; iPhone XR: 2942mAh
I remain unconvinced that other phones couldn't be engineered to have a user replaceable battery.
Water resistance is potentially a fair point, but iPhones have had non-user-replaceable batteries for far longer than they have been water resistant, and I'm not sure it would be impossible to do _both_ (Tile trackers have user replaceable batteries, and are water resistant up to IP55).