Here's why:

* Structure: Plastic molecules are made up of long chains of non-metallic atoms, primarily carbon and hydrogen. These chains are held together by weak bonds, making it difficult for electrons to flow freely.

* Electron Movement: In conductors, electrons can move easily from atom to atom, carrying electrical current. In insulators, electrons are tightly bound to the atoms, making it very difficult for them to move.

* Resistance: Plastics have a very high electrical resistance, meaning they resist the flow of electricity.

However, there are exceptions:

* Conductive Plastics: Some plastics are specifically engineered to be conductive. This is achieved by adding conductive fillers like carbon black or silver to the plastic matrix. These conductive plastics are used in applications like antistatic packaging and electronics.

* Electrostatic Discharge: While plastics themselves are insulators, they can build up static electricity. This is because friction can cause electrons to transfer to the surface of the plastic, creating a temporary charge.

So, while most plastics are insulators, it's important to consider the specific type of plastic and any additives it may contain.