I had an '86 Chevey Cavalier that was a year old when some drunk rear-ended it while I was at a friend's house one night. The police estimate he was doing at least 70 when he hit my car. The impact crushed it like an accordion, but the car didn't catch fire.
I wasn't in the car when he hit it - even though it didn't burn, I doubt I would have survived the impact had I have been. At the very least I would have been seriously maimed and/or crippled.
And there are photos of Vics which, after catastrophic rear-end collisions which did not burn either. I did not mean to speak of 100% of all incidents, but rather on the average. Possible variables in non-fire cases could be quantity of gas in the tank, relative humidity, angle of incident, etc.