why benzene dosent undergo electrophilic addition reaction?

In an addition reaction, electrons from the delocalised system would need to bond to the atom or group of atoms being added. This would result in a product less stable than benzene therefore not energetically favourable because saturating the carbon-carbon bonds destroys the delocalised pi electron cloud. Hence, benzene undergoes substitution reactions in which the product formed retains delocalisation, and hence stability of the benzene ring.