Plastic is usually scratchy. Glass is much better about resisting scratches. I believe you’re, right, modem phone glass is pretty flexible. It’s also (usually) coated with something that prevents finger oils from building up easily.
Still, the choice of glass is clearly cosmetic (less prone to scratch, nicer look, premium feel), buy technically plastic presents more advantages, it is lighter, absorbes shocks, fairly durable, much cheaper. To make for the most reliable device it would be better, but we know that’s not a priority for any of the manufacturers, and propably also for many customers.
Eh, I would argue that having a scratched screen goes beyond a cosmetic issue. It tangibly downgrades your experience.
Plastic scratches much easier than glass cracks.