And there's the problem that if the PIN or worse a biometric version of this becomes remotely well known you've now created an incentive for a car thief to kidnap people even if they only want the car.
A variant of this is the well-known scene from a previous century's movie involving extracting an eye to bypass retinal scanner.
The single incident was an individual in Malaysia who added an after-market finger-print based engine immobilizer to his car in 2005. The attackers threatened the individual with machetes, so that he would put his finger on the sensor, and then drove off with him in the car. The attackers later cut off his finger believing it would let them restart it without him present.