That’s the thing, half of all plastics in existence have been made since about 2000. They’ve just displaced everything else that we used to use in packaging, and lots more.
One of the biggest sources is clothing made of polyester. Fibers come off each time you wash it. Plastic wrapping might be the biggest- everything that is bought retail or wholesale in plastic wrapping is part of this.
It doesn’t get chemically broken down, but it does get scraped and ground down into really small pieces, and even the sun causes it to peel and chip and splinter on a micro level.