Since there is no such thing as black aluminum, the silver will show itself eventually when anodized. Some anodizing is better than others, but even mil spec HAIII anodizing can wear off. It can even flake off immediately after the process if the surface it was applied to had any dust particles or residues.
What's surprising isn't that there are some phones showing wear easily or even out of the box; it's that Apple decided to use such a relatively fragile finish in the first place.
If they really wanted us to have black phones, they could have used carbon fiber, fiberglass, or any number of dense hard resins. Or they could have used DLC coating instead of anodizing. DLC is pretty much the state of the art for making metal black, and tough.