In chlorobenzene a lone pair on chlorine overlaps with the delocalised pi system of the benzene ring, producing resonance structures in which the carbon-chlorine bond gains partial double-bond character. This shortens and strengthens the bond, making the C-Cl bond harder to break and reducing the ability of chlorine to leave as a chloride ion. The sp2 hybridisation of the ring carbon also holds the bonding electrons closer to carbon, reinforcing the effect. The steric-bulk option is wrong because the inertness is electronic, arising from resonance, not from crowding by the ring. Lower electronegativity of aromatic carbon is incorrect; sp2 carbon is in fact more electronegative than sp3 carbon. A stable benzylic carbocation is irrelevant because no such cation forms; the halogen is on the ring, not on a benzylic side chain. Because of this combination of partial double-bond character and sp2 hybridisation, chlorobenzene reacts with nucleophiles only under very harsh conditions, such as the high temperature and pressure used in the Dow process for phenol, or in the presence of strong electron-withdrawing groups ortho or para to the halogen. The same resonance also makes the molecule less polar than expected and lowers its dipole moment relative to an alkyl chloride. This is the NCERT explanation for the low reactivity of haloarenes. Sanity check: partial double-bond character implies a stronger, shorter bond, consistent with poor leaving-group behaviour.