Why cannot 3-phase induction motor run at synchronous ‘speed?

The rotor follows the stator field. In actual practice, rotor, can never reach the speed of stator field (i.e. synchronous speed). If it did, there would be no relative movement between the stator field and rotor conductors and, therefore, no torque to drive the motor. Hence 3-phase induction motor can never run at synchronous speed.