The thing is that even if there isn’t much energy in plastic to be extracted, theres still enough energy in it to make a viable food source. Now, consider the humble panda and its primary food source, fucking eucalyptus leaves. Theyre so hard to chew that koalas had to spend evolutionary time and energy just to spec into it to the point they cant eat anything else pretty much. Combine that with the fact that eucalyptis leaves are so devoid of nutrients that the koala has to spend all day every day just snacking on them to not die of malnutrition.

Why? Why would a species even bother with this flim-flam if eucalypti sucks that bad as a food source? The answer is: Food scarcity. Because eucalytis grows everywhere where koalas live and because nobody else is bothering to tap into the food source, this sets up a ecological niche by pretty much gaurenteeing any animal that sucessfully finds a way to make it work will have unlimited amounts of food/energy just from the fact theres so damn much of it and nothing else wants to/can touch it. Sure koalas might have paid the price by sacrificing some brain wrinkles but who needs higher intelligence when you have leaves to snack on and sex to make babies.

A similar thing happened with trees and mushrooms. In the deep evolutionary history of our planet trees were once the apex forms of life with forest covering pretty much the whole planet. This is because nothing knew how to break down the wood making up stems for a good couple million years. Most of the coal and oil that we dig up today is actually the preserved remains of these unbroken down trees from the carboniferous period that just layed there petrified never rotting until the carbon compressed into hard rock or squeezed into liquid. The great change in the era happened when our humble mycelium bois finally figured out how to eat wood, causing them to essentially become the new apex life for a time.