Why can't plastic decompose?

Plastic is designed to be inert and stable so it is no surprise that it is slow to decompose

The conditions for plastic decomposition are usually limited to attack by ultraviolet light or heat which break down the bonds between the monomers and cause the large plastic object to shatter and reshatter until it becomes just an inert particulate.

Plastic decomposition under biological attack is limited as few bacteria find it "tasty" however as some bacteria will eat crude oils there is undoubtedly some species that can be raised that will feed on plastic.

As far as plastic decomposition in waste disposal sites, these facilities are designed to minimize decomposition to prevent methane generation. They are both dry and anaerobic. Eve sfoodstuffs (fruit and steaks) have been found preserved after several decades of burial.