Because the phone is water
resistant (not waterproof), which means that it it's raining and the phone is in a pocket that gets wet, or if it's
very carefully placed in water, the water will stay out. "Washing" the phone causes enough pressure to force water into the phone, and I'm afraid that by now it's too late to do anything to save it. It may work now, but it's slowly going to degrade until it stops working. Read
Oh, no! My Phone got Wet! (And rice has destroyed ore phones than dropping them, sitting on them and hitting them with a hammer combined. It's absolutely
not something to do to a phone - it only makes the problem worse.)
Don't be disappointed with Sony, be disappointed with your misunderstanding of "water resistant". That's okay though -
most people think it means waterproof.