Why is it so hard to turn plastics into something useful or decompose it? Even after so many years, why haven't we yet come up with a solution to the plastic problem ?

Why are plastics so hard to decompose via chemical or physical processes into something harmless or useful ?
Most plastics are reusable . The problem is wastes are mixes of various different plastics and other materials . The cost to reliably separate them into specific materials is greater than the cost to make new plastic.