Why is a molecules with high energy density so difficult to break down?
Why are there so few natural decomposers of plastics?
Is this because bacteria haven't evolved the correct enzymes, and many other non plastics globally have bacteria consumers that we can just assume that they decompose naturally?
Yet. The results might not be that great for humanity either. Especially if the bacteria get, shall we say, overly eager and efficient?... and plentiful.
That said, there's plenty of obscure bacteria having a decent stab at the problem and having a good time. Humans are helping them along as well.
Like all decently powerful tools there's a possibility for harm and hope.
Other than climate change, I think plastic pollution is the issue of our time.
Because plastics weren't naturally occurring. Give it time.
Sometimes nature is very slow. Like geological timescales.
https://www.researchgate.net/figure/Chemical-structure-of-li...
Generally, even if a given reaction is energetically favourable, if the activation energy or enabling conditions for the reaction to occur are high/rare respectively, then the rate of the energetically favourable reaction will be slow.
It's like water in a cup; sure it would prefer to be on the ground, but absent something tipping the cup over, it'll stay inside the current local energy minima without any complaints.