One of the big issues with recycling plastics is that plastic has a very specific chemical makeup that gives it the properties it requires, and one major way to mess up that composition is heat. So, even if you can perfectly sort plastics into their respective types, simply heating them up to re-cast into pellets or something else can affect their properties to a state that they’re not usable anymore. Add on top the fact that you will not be able to perfectly sort plastics by composition so you will always end up with a significant amount of impurity makes recycling very difficult.