There are a lot of different plastics with different properties, but the main reason plastics are non-biodegradeable is because the chains of molecules called polymers that make up the plastic are too long and the bonds between the molecules are very strong to be broken down. So they will eventually be broken down, but it takes a lot longer than biodegradable materials.
Some plastics today are biodegradable. They are made from things other than petroleum products. Polyhydroxy alkanoate (PHA) biopolymer is a completely biodegradable plastic. PHA's are linear polyesters produced by bacterial fermentation of sugar or lipids. There are other bioplastics made from things like starch, natural fats and organic oils.
It doesn't decay because of its quality