Plastics generally have a bunch of other things added for a number of reasons.
One of note is a plasticizers, which are smaller molecules that basically soften the plastic by preventing the molecular chains from folding upon each other as tightly as possible. They can leach out over time, resulting in the material becoming more brittle and a greasy film covering the plastic (though this takes several years).
Plasticizers, the molecular chains themselves, and other additives (fillers, heat stabilizers) can also degrade with time (especially in the presence of heat or light), resulting in yellowing (typically apparent in white materials, but can be observed in others).