it really should be. Most people don't know the difference between IP67 and IP68 and think that the iPhone 7 will be fine if you throw it in a pool. I repair iPhones for a living and recently saw a liquid damaged iPhone 7 that was the worst liquid damaged phone I have ever seen.
I know you can take the view that people should know better and not deliberately immerse their phone in water but if you call a phone "water resistant" people will as the average person just doesn't know the difference between that and full blown water proof
I'm not going to pretend I know the costings but I would think for what apple charges for the iPhones they could have made it IP68 to save the average person from loosing their phone (And their data! )