[Retric]

More that the don’t break down fast enough. Most individual plastic molecules on their own doesn’t last that long. Polyethylene the most common plastic is simply a very long chain of carbon and hydrogen it’s a ready food sources for many different kinds of bacteria and is broken down by sunlight etc.

There are of course more and less chemically stable plastics, but they all last much longer in a landfill than the vastly more harsh aquatic environment.