Here's the problem: Take a biodegradable bag and grind it up into dust, then sprinkle it on the ground-- no problem. Bacteria in the soil will eat it and turn it into biomass and gas. Do the same with a plastic bag, and the tiny bits of plastic will just stick around, and plants grown in that soil will be contaminated by plastic per the study.

These bags aren't breaking down quickly simply because they are big. Decomposition is like any other kind of eating; you keep breaking the stuff down into smaller bits until it's gas and biomass. But with plastic, where biodegradation can't happen, the tiny bits (nanoplastics) accumulate.

Fun fact about nanoplastics: we can't reliably measure them! We can make a reference solution and apply it to arabidopsis, but we can't count nanoplastic particles in wastewater. I'm told they can be as small as a protein molecule.