These caterpillars didn't evolve to eat plastic as far as we know. Evolution takes place on unimaginably slow timescales. It took tens of millions of years for things to evolve to eat wood. Probably these caterpillars already happened to produce a chemical that reacts with plastic. I wonder if they even get any nutritional value from it.

That said, bacteria can evolve pretty fast because of high population sizes and low generation times. In the 50's they found a bacteria that had evolved to digest the synthetic material nylon. I wonder why bacteria haven't evolved to digest plastic yet. Possibly there is some scientific reason that it's inefficient to digest. Maybe we could speed evolution up by dissolving plastic in a liquid first so it's more accessible to them.