This was mentioned before. Plastic is used in all sorts of applications for its durability and lack of reactivity. It doesn't rot. It doesn't corrode. In certain environments it dries out and cracks but that's about it.
While there's a lot of plastic in the ocean it's relatively thinly distributed. It's not like there's a bunch of bottles floating in one place. It's a bunch of plastic dust in the water column. Any sort of bacteria or chemical that is self sustaining enough to be dropped in the ocean and expected to clean up the plastic would likely wreak havoc on all sorts of things. Any sort of bacteria or chemical that is sensitive enough that it won't wreak havoc in the environments where we don't want it to likely can't be applied in a effective or economical manner in the oceans.
While there's a lot of plastic in the ocean it's relatively thinly distributed. It's not like there's a bunch of bottles floating in one place. It's a bunch of plastic dust in the water column. Any sort of bacteria or chemical that is self sustaining enough to be dropped in the ocean and expected to clean up the plastic would likely wreak havoc on all sorts of things. Any sort of bacteria or chemical that is sensitive enough that it won't wreak havoc in the environments where we don't want it to likely can't be applied in a effective or economical manner in the oceans.