The reason
why is due to all the hackers out there that got their iPhones bricked. Many were putting thier phones in water, waiting for them to dry, then taking them back to the store saying it didn't work (but never telling them they hacked it). Apple on many new ones added a water damage sticker (changes colors when gets wet and stays the new color when dried) so if it shows it was wet, they don't fix it. For you, you say you had it, did nothing with it, and it got wet and now won't work. To them, it just sounds like a hacked phone trying to get a new one by doing what many hackers posted as a way to get a new one but no longer is a valid way.
Now I am in no way saying you hacked it and are making this up, but rather pointing out, the fact you did not activate it and had it so long and now come in with water damage, to them, it is the classic, bought the phone, hacked it, messed up, now trying to get a new one.