I am not the right market for flagship phones because I don't have thousands of dollars to put towards disposable income, but I would expect a premium phone to come with a premium warranty and repair service... Do they typically not?
AppleCare specifically covers water damage of any kind, and is what I would assume OP would be referring to as a "premium warranty and repair service".
ApplecCare is as much a warranty as their Apple's phone are guaranteed waterproof.
It's marketing.
The 3 year warranty add-ons is an insurance. One can get an insurance for any device.
The standard iPhone warranty disclaims water damage [1] and Apple has been sued in the past for using humidity activated sensors.