Why does apple warranty not cover iphone getting wet in normal use ?

I don’t know of any manufacturer of IP rated water resistant smart phones whose warranty covers water damage. None do to my knowledge. Water resistance is not a guarantee that damage will not nor cannot occur. It just helps reduce the likelihood of damage relative to a non-water Rostand rated device under similar circumstances.

Also note that the water resistance degrades over time, so water damage becomes ever more likely, if exposed, the longer you own the device.

I think some models of Cat ruggedized smart phones cover water damage, but only if the seals were in place over all ports, microphones and speakers (unlike most smart phones, Cat phones use rubber or silicon plugs on all the exposed openings).

No one here can tell you the "why" Apple does anything ... however, we are all under the same product warranty that Apple provides us. In that warranty Apple specifically calls out that they do not cover any types of liquid damage. Only their AppleCare+ plan offers some financial relief for a phone replacement. It just may be that Apple deems "normal" use different than you or me.

Ref:

• Apple One (1) Year Limited Warranty - Apple
• Water and other liquid damage to iPhone or iPod isn't covered by warranty - Apple Support