Maybe a lot of time. The main reason we have cheap coal is from nature failing to find a solution to break down lignin during the Carboniferous period (and really particularly during the Carboniferous rainforest collapse) hundreds of millions of years ago.

Sometimes nature is very slow. Like geological timescales.

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...