I find it disappointing when people accept this as normal on a supposedly premium device.
Apple should use more durable materials
This is a poor assertion on your behalf. iPhones are some of the most premium and well made phones on the market, which is not surprising to many. It comes down to the fact on how one takes care of their phone. As an iPhone owner who has plenty of experience with many models, their displays, casings and overall construction are excellent.
Furthermore, surgical stainless steel/316 L stainless steel naturally will scratch easily, because it's a very dense metal. It's not just about the iPhone, it's about the metal properties in general that are susceptible to scratching.
My Apple Watch users 316 L stainless steel, but it's by far from being not durable because it scratches. It's the nature of the metal.