Edit: I was replaying to the wrong message, clicking is hard.
Sorting is way easier if you have to just pick stuff (semi automatically) from a conveyor belt vs having to pick individual object and separate the plastics (first figuring it how to do that, and also objects are all damaged).
Like plastic bottles and plastic bottles caps - it would be (or it partially is) immensely costlier to separate them by employees at the sorting site vs each of us taking the the ... or not taking the time end effort to screw the cap back on.