It’s not really a benefit if the original carbon came from oil, because the carbon that was originally deep underground is now at the surface. If anything it’s only neutral, however the carbon emitted during oil extraction and the manufacturing process still results in a net negative.
Landfill isn’t the worst option, but it’s still better to try to avoid the manufacturing of it to begin with.
Although you are right, I don't think it is that people don't know this. Recycle is easy, through it in the blue bucket. Reduce requires a complete re-imaging of our society. This won't happen without substantial taxation on plastics and single use packaging. But that is a bad way to get re-elected and how will we get next day shippping?
To go a bit further, I feel like "reduce" just isn't really in the vocabulary of living organisms. Once any particular cat is out of its bag, it's not going back in (though I'd love to hear some examples to the contrary). It seems like our best bet really is an ever more Rube Goldberg-ian stack of attempted fixes for the problems we create.
The answer is simple mechanically (let economics work by pricing in externalities) but it’s politically tricky because an established industry has significant lobbying and propaganda clout.
There are success stories such as the sharp reduction of CFCs and acid rain contributors, or the reduction in coal pollution once it cost more than alternatives, but we can see the opposite in, for example, the way the oil industry successfully captured a significant fraction of a major American political party and got enough people to believe climate change wasn’t real to avoid a carbon tax for decades we could have used to transform the economy less sharply than will now be needed.
Modern cultures have substantially reduced their use of tobacco and alcohol, have reduced their birthrate below replacement, cut 95% of trans fats out of their diets, and use practically no tetraethyl lead and no DDT at all.
We don't have to continue using endocrine disrupting plastics either. It can't be a blanket ban, we use them in too many places. But they can be phased out.
Landfill isn’t the worst option, but it’s still better to try to avoid the manufacturing of it to begin with.