LOL she is she Asian?
Seems common in Asian household

I mean to be honestly, there's no reason to keep the plastic unless she's planning to resell it. Otherwise if her intention is to keep it from scratches so it looks "good", then obviously it's not working because it looks horrible

Ask her WHY she's keeping it, maybe you can talk her out of it.
Oh and "My girlfriend won't take it off" is a very misleading topic. Unless you're having trouble in that area too lol.