Why dont they decompose?
Since plastics belong to a chemical family of high polymers they are essentially made up of a long chain of molecules containing repeated units of carbon atoms. Because of this inherent molecular stability (high molecular weight), plastics do not easily breakdown into simpler components. However plastics do decompose though not fully and that too over a very long period of time (100 to 500 years). Commercially available plastics (polyolefins like polyethylene,polypropylene etc) have been further made resistant to decomposition by means of additional stabilizers like anti oxidants.