I guess I’ve always assumed the main reason people use iPhone cases is to keep the glass and camera lenses from being destroyed if you drop it.
Same with the iPad, given that an entire side of it is a massive sheet of glass, but also with the functional benefit of different viewing angles.
With a Mac, even a laptop is usually semi stationary, and when it’s being moved from one place to another it’s folded in such a way where all of the very breakable stuff is protected and the outside is just aluminum.
Plastic over the aluminum seems completely redundant to me, a sleeve will be much more protective against the drops and if you’re trying to avoid scratches, the shell could actually do the opposite and introduce more.