Coatings can chip and can scratch, and aluminum, titanium, and other metals used in an iPhone and elsewhere are not known for integrating color throughout the full depth of the material.
Same issues for vehicles and vehicle paints, for buildings and paints, and any number of other coatings found in modern usage.
Figure out how to do that color integration both cost-effectively and without materially diminishing the necessary properties, and make yourself a fortune.
Otherwise, these are coatings, and damage can remove the coating.
Proper protective cases can help reduce the damage to coatings, and to the device itself.
Otherwise, the devices will accrue a patina.