Given a relatively high and sufficiently uniform clamping pressure on the entire o-ring. Which then requires high-stiffness parts (bulky, heavy, high material usage) or a lot of fasteners (take up space, potentially add to the oh-so-hated bezel width depending on how they have to be implemented).
Actually, no, this is not necessary for 1atm water resistance. See: the Samsung Galaxy S5 that was made out of ABS plastic, and had an easily removable battery with 0 fasteners. It was IP67 rated.
O-rings are the standard seal technology in waterproof watches, and also in scuba diving equipment where lives literally depend on them. They are actually extraordinarily reliable.
But at the same time, a diver is trained to look for leaks as part of their pre-divw checklist, and oring replacements are fairly common. On my last liveaboard, the crew replaced probably good 1-2 dozen tank orings over the course of a week. One of my LP hose orings had frayed as well and needed to be replaced.
It might be the standard, but it's not exactly the best experience either.
The o-rings that require regular attention are the ones that handle high-pressure air. Run-of-the-mill waterproofing o-rings, like the ones used to seal dive watches and dive computers, are fire-and-forget. And yes, dive watches and dive computer have beefier housings than iPhones, but iPhones don't need to be waterproof to 150m.
Dive computers with replaceable batteries 100% have user-serviceable o-rings that need lubing and eventual replacement. My Shearwater came with both grease and 2 replacement o-rings.
HP hoses and o-rings also can and do fail, and, once again, divers are trained to look for signs like the rubber bubbling. There is also not really a difference between the o-rings used in HP and LP hoses and equipment outside the size, and I gave examples of LP hoses failing.
These all require regular maintenance and replacement, and, once again, divers are trained and should be checking their equipment regularly.
But that's all beside the point, which is that your claim is that "[o-rings] are extraordinarily reliable". Except they are not, they require replacing all the time. They may be industry standard, but they are fall from infallible.
Scuba equipment is most often used in saltwater and is subjected to increased pressure while underwater. The conditions scuba equipment is subjected to vs what waterproof cell phones are rated for are very different.
Chlorine isn't friendly to o-rings, but I'm not sure where someone would put their phone while in a pool anyways. Mine comes in the tub with me for soaks for sure though.
Do o-rings have a dramatically high failure rate that I don't know about? It can dry rot or otherwise break down, but as a seal on the back of a phone, I've found them 100% reliable.
So would a rubber o-ring.