Because plastic is fiddly to dispose of, and makes a mess when you just leave it lying around. Uncoated paper tends to disintegrate into safe mush, which allow

Because plastic is fiddly to dispose of, and makes a mess when you just leave it lying around. Uncoated paper tends to disintegrate into safe mush, which allows you to dispose of it without much care, including by sticking it in a heap somewhere and forgetting about it. In a pinch, it also burns relatively cleanly, and doesn't gum up shredding/bundling equipment.