An honest question!
Plastics are polymers. The monomers, such as propylene, are reacted over catalyst so that they develop long chains and "cross-links" to give the polymer some rigidity.
When exposed to the atmosphere and in particular the sunshine, the polymer chains are damaged by UV light and/ or oxygen from the air.
So, manufacturers of uPVC windows use anti-UV chemicals in their profile extruders.
(by the way, uPVC is unplasticised PVC).
Similarly with polythene, PP, PET, etc.
Plastics can be re-used, as in India where they use PP flagons for water storage/ transport.
Plastics can be recycled, as in UK where they are chipped-up and mixed with fresh material before "new" bottle manufacture. (But, the new bottles are not quite as good as "virgin" bottles)
PET bottles can be chipped and subjected to "solid phase polymerisation" to get rid of the moisture which otherwise causes degradation of the recycled product. This is expensive, consuming capital plant, people-power, and fuel.
Is there a better way, such as de-polymerisation, which regenerates the monomers, which would then have to be separated by distillation?
Then, we wouldn't need to send any plastics to landfill.
Ideas for Green Earth