They removed 3 ways water could get inside the phone
plastics dont break, are cheap to replace, don’t get hot or cold with weather, don’t interfere with wireless charging, have much more grip, and can be made into any texture (matt or glassy).
I don’t understand why anyone would prefer metal/glass phone besides for the reason “look at me, ma, my phone is pretty”.
Personally, water damage on the phones I had always was a rare edge case. The only time I ever water damaged my phone was when I accidentally put it in the washing machine and I’m sure no phone would have survived that.