A quickly decomposing plastic wouldn't be awesome either as it would add all the carbon extracted from fossil sources into the short term carbon cycle more quickly.
Compostable "plastics", e.g. PLA, which sometimes is corn-based, exist; in California I've seen compostable disposable silverware, as well as compostable decorative planters (I own two, holding up good after three years, which makes me wonder how compostable they really are). But compostable "plastics" do exist, at least in some markets.
Surely the companies that print the symbol and "Please recycle!" on non-recyclable plastic wouldn't label non-compostable silverware as compostable...
I've successfully run "compostable" plastic through the dishwasher several times and haven't seen any sign of decay. The county I live in offers composting service and begs people to please not put "compostable" plastic in it.
Of course it makes sense. Plastic is cheap, light, watertight, strong, and mallable.
It's kind of a miracle material, except that it's too stable. If we had a version that decomposed in a year, it would be awesome.