Many plastics are indeed organic compounds. Polyolefins (polyethylene, polypropylene, etc including their derivatives), nylon and its derivatives, vinyl and its derivatives, for example, are all organic compounds.
Silicones are NOT organic compounds.
But silicones move so well. /sarc
Silicones are degraded by bacteria in the soil and water. They metabolize the methyl and other organic groups leaving silica (ie sand). Silicones’ use however is limited mainly to elastomers and pressure sensitive adhesives.
Nylons will biodegrade since their structure is somewhat like a protein (don’t bury your Glock in the ground without wrapping it; it won’t be there after a while).
The real culprits are polyolefins because of their extensive use in packaging. Researchers (my professor from school) have looked into substituting polyvinyl alcohol, polyhydroxyalkanoates and the like because they biodegrade.
The problem is if you add even a fraction of a cent onto packaging costs management balks.
(see my username)