I was under the impression that Americans landfilled their garbage. I don't doubt that waste export is a thing, but is it really that large a proportion?
The US is the only country in the developed world that hasn't ratified the Basel Convention [0] and the US exports close to a billion tons of plastic waste to other countries per year [1]. The trash you see building up in third world countries is in large part from the United States, not from those countries themselves.