Wood is a biological material made from molecules that have been sitting in Earth's biosphere for hundreds of millions of years:
Wood
→ cellulose
→ hemicellulose
→ lignin
→ sugars/aromatic compounds
→ microbial metabolism
Those molecules contain lots of chemically accessible bonds, especially C–O bonds. Plants built them using biological chemistry, so other biological chemistry can, in principle, take them apart.
An enzyme isn't a tiny animal chewing through plastic. It has to contact a polymer chain, bind it in an active site, chemically cleave a bond, release the products, and repeat.
A plastic can thwart that by being:
hydrophobic + highly crystalline + insoluble + chemically repetitive + physically inaccessible.
There's no evolutionary law saying organisms must eventually become capable of rapidly consuming any energetically favorable substance.
Evolution has to find a chemically feasible pathway. And even if it does, decomposition might be:
plastic bottle → 150 years
rather than
plastic bottle → three weeks.
And amusingly, we could simply change the plastic. If PET suddenly became unsuitable for decades-long applications because PET-eating microbes became ubiquitous, we'd manufacture polymers resistant to those enzymes, add protective layers, alter crystallinity, or use different polymers.
All and all, I’m doubtful microbes will come to our rescue. Unless we engineer them for the task, that is.