So the claim that "it all ends up in a landfill" - may not be completely accurate.
Requiring residents to sort recyclables means that the companies who use recycled plastic to make these products don't have to - that most of their work in this initial step is done for them.
It might be claimed that they get it from the ocean. Then the transportation costs would probably be higher. So it's more likely that they use domestic plastics provided by residential waste collectors - that's already sorted.
I have NEVER recycled one thing.