Why are plastics considered non-biodegradable?
Plants,, animal bodies all consumed by some form of bacteria and get decomposed.
But plastic materials can not be consumed by such earth bacteria. They remain on earth for thousands of years.
Most plastic is composed of polymers derived from ancient vegetation which become exceptionally long molecules when heated in the manufacturing process, producing a product that is unnatural.
Plastic is made from petroleum, a product of oil, using heat and a catalyst to change the propylene into polypropylene, a substance not found in nature. PET is a polyester. Since these are unnatural products, not found in nature, there are no organisms capable of decomposing the material, so it will not degrade as does other plant and animal waste.
Plastics present many problems beside being non-biodegradable. These include: they are flammable, pollute the environment at a staggering rate, and cannot be safely burned. Plastics are killing wildlife when they are mistaken for food, and stick in the body until the bird or animal dies of starvation.
There are some plastics that are processed to avoid the strong carbon bonds formed in the heating process, but they will not last as long as the mainstream plastics, and will start to break down much sooner.
Information on polymers used to make plastic is here: https://www.livescience.com/33085-petroleum-derived-plastic-non-biodegradable.html
Consider the source of your drink:
Plastic bottles can be recycled: