It depends on what plastics we are talking about. Some plastics are designed to degrade in a few months (i.e certain plastic bags) while some last a lot longer.
Polycarbonate is used in a number of ways, not sure how the casing in the 5C has lasted, but since its being used in all from the aircraft industry to making blueray discs.
One other reason why it isn't used might be recycling, metals could be easier to recycle than a polycarbonate case.
Sure some of the more recent carrier bags are designed to be biodegradable and break down quickly but then you’re not going to use that type of plastic to make a phone are you!