I'd say a major difference is that steel is an alloy of natural inorganic elements. The components aren't especially toxic, at least not any more than how they already exist in the environment. Microplastics are entirely man made and don't have any natural, decent way to be broken down organically. Would you count sand as a pollutant? It'll last way longer than plastic and we certainly ship it around everywhere. Eating sand is just part of marine life