Screen malfunctioning again

I took my iPhone XS into the apple store again in less than a year for the screen malfunctioning. They briefly looked at it and acknowledged that there was a problem with the screen and then informed me that there is water damage and that they would not fix it but replace it for a fairly high dollar amount. My problem with this is that the phone has never been dropped into water so how is it possible for water damage to occur? Not to mention these purple lines appearing on the phone for the second time within a year is ridiculous. I have been a long time Apple user and have an abundance of Apple products in my home and I have never been so unhappy with a product. First picture is from June and the second from January. If someone could please get back to me and explain how a phone that has never been in water can have water damage I would really appreciate it.