It is because, since 1995 it has been a legal requirement that vehicles are fitted with an immobiliser system. To engage or deactivate the immobiliser, a transponder chip is fitted into the top of the car key or transponder key. The microchip contains a serial number that is unique to that key.
The transponder key is programmed to transmit a signal to the engine control unit and the immobiliser box and allows the car to start when it is inserted into the ignition. The key is programmed to start only one specific car, the one it was purchased with. A replacement or repaired key must be programmed using the same serial number so that it can signal and receive the correlating response, otherwise, any attempts to start the car will fail.