Steel and Glass are for skyscrapers and cars. Phones need to be plastic again to reduce their weight and cost. They're not meant to be luxury items but handy pocket computers. Nothing devices and Samsung's FE series all fell great in the hand and are both plastic.

The real reason most poeple use a case is because phones are badly designed products. Larger displays were pursued in the name of content consumption, not utility. Software was never redesigned to accomodate, only stretched to fit.

A phone case is needed because phones are made from fragile materials and because too many interactions involve the top 2/3rds of the display.