The US produces plenty of plastic waste on its own, but it ends up in landfills because of modern garbage collection and street sweeping infrastructure. Before the development of landfills and garbage trucks, trash was a much bigger problem in the developed world - plastics just weren’t very common yet.

Most developing countries don’t have that infrastructure so plastic pollution is everywhere, regardless of how much they export to Western countries.

It's not "regardless" at all though. Knowing they don't have the infrastructure in place to deal with existing waste, it's easy to predict what happens when we outsource production there. However much uncontrolled waste they would generate, we are choosing to add to it.

Most of the waste likely comes from their native consumption, not their production.

Not much? It’s the habits of the society. Other places have similar economic situations without the pollution.