My assumption is that unscrupulous recyclers are shoving it into rivers where it flows to oceans and ends up on beaches. Not all recyclers are honestly separating types, chopping it up and reselling as pellets.
I am suspecting the ships find out the plastic processors are maxed out and dump the plastic at sea. They are not going to tie up a ship because there is no place to for the containers full of recycled plastic. I bet this has been going on for years.