Because that tends to only happen in a few, what we consider, first world nations. A huge portion of the world does not have sanitary waste collection system, and therefore produces huge amounts of plastic waste that get into rivers and the ocean.

Of even in places with waste collection, an (un)healthy amount of plastics never make it to the trash. Blow off/run off from trash cans is one example. Another is direct environmental loss into the environment. For example washing your clothes releases massive amounts of plastics, unless you happen to buy only natural materials.

Plastic is both a wonder material, and a wonder mess. Using it in things like medical device sanitation is a net benefit for humankind, using it everywhere appears to have higher costs than expected.