Plastic does break down, it is just really slow. But since the oil was extracted from thousands of feet down and was already sequestered before we decided to ma

Plastic does break down, it is just really slow. But since the oil was extracted from thousands of feet down and was already sequestered before we decided to manufacture stuff with it, it can only at best be carbon neutral, assuming the plastic lasts as long as the original oil would have. You could make it carbon negative if you converted CO2 gas into plastic, but the energy requirements to do so are enormous. It is the same problem with fertilizer production. We could make synthetic fertilizer from the air, but it requires so much more energy to the already immensely energy intensive production process that you don't actually gain anything. If the energy needed was all clean energy, and we had enough of it, we could make carbon negative plastics. But that is like ridiculously cheap electricity to the point where residential power costs would be measured in like 50 cents per year, minus power line upkeep.