> Isn’t plastic nearly free as a raw material as a by product of fossil fuel production?

Yes and no. Oil is cracked down for lots of different things, but one of the things that people are really aiming for is ethane which can be used to make industrial ethanol, and a lot of plastics.

What you end up with to get there is lots of propane and butane, which is used as a fuel gas, but most of that ends up going up the flare stack because there's so much of it. We won't stop doing this while we still use plastics and fertiliser.

You need very clean propane or butane for things like cooking stoves, but you can run car engines on it just fine because they're so much less picky, and the exhaust gases are far cleaner than engines run on heavier fractions like oh maybe octane. This is why things like warehouse forklifts run on gas instead of diesel or petrol - you don't die if you breathe in the fumes, because there's no unburnt fuel or carbon monoxide, because it runs as lean as a vegan's dog.