The easiest way to get an IP rating is to use lots of glue. If they went the o-ring route, it would take up more volume and would probably require replacement

The easiest way to get an IP rating is to use lots of glue. If they went the o-ring route, it would take up more volume and would probably require replacement when the phone was opened as damage from dirt/age will make reusing the o-ring hard. Even worse is that resuse isn’t obvious to a layperson as it could be IP69 with a new o-ring and IP67 with reused o-ring.