I don't see why people drive off. The police will catch you and the punishment will be worse (questionably)
How very old fashioned of you, Dan
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The driver was probably drunk/drugged; driving with no licence/MOT/insurance, disqualified from driving, and in a stolen car (or his mates which he had "borrowed" without mate knowing about it, of course).
He knew fine well that his chances of getting caught if he left the scene were slim as long as no-one got the number of the car. Wouldn't be surprised if the car is eventually found burnt out, having been reported stolen after the incident.
Cynical; me?