Lignin can be broken down in aerobic conditions by peroxidase and a series of other fungal enzymes, it only forms those energy dense forms under anaerobic conditions and probably requires high pressures. The reason it is so difficult and only breaks down through oxidation is because it is quite complex, unlike the homogenous polymer chains of plastic that have many repeating monomers.

https://www.researchgate.net/figure/Chemical-structure-of-li...