Their journey ends in China, where they're made into slipper soles, among other things:

To be sure, it's possible to shred wire in the United States. But unlike China, where there are plenty of manufacturers eager to buy large volumes of rubber and plastic insulation, the United States lacks such industrial demand, forcing U.S. recyclers to either landfill insulation or sell it to power plants as fuel. But the lack of a U.S. market for chopped plastic and mixed chopped copper and brass creates a counter-intuitive (for American environmentalists, at least) result: not only do Chinese recyclers recover more material from Christmas tree lights than Americans, they make more money, too. After all, they can sell the insulation, not pay for its interment.