It's also partly because phones now require 60,000 antennae and radio waves don't go through metal. Wireless charging, NFC, wifi (x2), bluetooth, cellular (x4), UWB..... There's some ability to reuse the antennas via TDM and other tricks but they just "need" so many these days. Also also, plastic is kinda evil from a pollution standpoint, although one could also argue that it could just be recycled with the rest of the phone.