At the end of the day there is no combination of materials that Apple could use, maintaining the size and form factor of the iPhone, that would reliably survive drops or rough daily use without at least significant cosmetic damage, ergo you need to put a case on it if you want to keep it nice.
Aluminium bends, titanium is a lot more prone to scratching than people think, glass breaks, plastic snaps, any metal on the back would block the wireless charging, etc etc.
Stainless steel is at least good and rigid, improving the structural integrity of the phone and protecting the glass to some degree, and it provides a shiny surface that’s easy to grip even if the back glass isn’t (titanium would be less good for that). It’s also nice to look at, until you do the only sensible thing and put it in a case.
Aluminium bends, titanium is a lot more prone to scratching than people think, glass breaks, plastic snaps, any metal on the back would block the wireless charging, etc etc.
Stainless steel is at least good and rigid, improving the structural integrity of the phone and protecting the glass to some degree, and it provides a shiny surface that’s easy to grip even if the back glass isn’t (titanium would be less good for that). It’s also nice to look at, until you do the only sensible thing and put it in a case.