From a mechanical perspective ip68 is perfectly achievable mechanically and watches have been achieving it for a long time, however… with what sort of margins for the manufacturer and what sort of cost for the consumer ? Additionally a lot of them require pretty carefully adherence to instructions torques and tolerances to achieve the same waterproof rating. Personally I’d be very happy to have a phone that says, if you swap the battery you might lose the ip68 rating unless you follow the resealing process within tolerances.

My phone (A Furiphone FLX1, which is kindof a variant of a Gigaset GX6) has a removable back with a gasket and is IP68. One of their promotional videos had them change the battery on video then boot the phone and and unlock it underwater

Who cares though? Sealing the battery in makes the device less drop resistant. I somehow managed to avoid water damage to my phones for decades, while none of my phones managed to avoid being dropped in a way that would most likely be fatal to them if their batteries were sealed in - and yet most of them survived to this day.

A phone needs to handle some rain droplets falling on its screen, anything more than that is a gimmick that's not worth the downsides it comes with.

Nothing stops them from adding a gasket and some screws though.

Maybe as a society it's better for people to have replacement insurance than to have sealed batteries that make phones so disposable. I wonder if we've defined IP68 as a "must have" without considering the alternatives. I'm thinking the percentage of people who actually "use" IP68 over the course of their phone is pretty small...yet that "requirement" drives a huge design choice.

I suspect it's a moot point. Makers have every incentive to drive replacement cycles.