Metal won’t work because wireless charging is expected for most phones

It’s not an impossible problem to solve, Qi just doesn’t care. Qualcomm solved it with their wireless charging standard 10 years ago

Leaves glass and plastic, which are both found on premium phones.

I don’t think I’ve seen a “premium” phone with a plastic back in a LONG time

it feels and looks nicer than plastic and doesn’t scratch as easily

There’s a wide range of plastics, it gets a bad rap because of the cheap shit that’s often used, but there’s absolutely a “high end” plastic side of the scale that can beat glass and not nearly as fragile