A lot of bioplastics turn out not to be so green if you run a full lifecycle analysis.

Appart from that: Plastic has this somewhat unique property that nobody is really held responsible for its emissions, even in places where emissions are regulated. E.g. the chemical companies are usually only considered responsible for the emissions of their plants, while the bulk of emissions happens on landfills or in incinerators. Waste-to-Energy-Incinerators on the other hand are in many places even considered "green" energy, because they say we're only using waste that's already there.