There was a lot of discussion about various aspects of how cars behave when they crash, in a previous thread. But, speaking as a former firefighter, the tl/dr; is "cars do sometimes catch fire and burn when they crash". Hollywood style explosions don't normally happen, and car fires resulting from wrecks don't always happen, or even happen terribly often at all in my experience, but they are far from unheard of.
Here's a report of a Mercedes crashed at Nurburgring by a test driver: http://www.automotto.com/2014-mercedes-benz-sls-amg-black-se...
Both the test driver and the passenger walked away unharmed. Do note that this was on the track, where the cars are supposedly driven at much higher speeds than on regular roads.
No, scarcity isn't proof against, but it is evidence. A lot of people are talking nonstop about the rarity of cars bursting into flames as evidence against that interpretation, while ignoring the even greater rarity of journalists being murdered by the US government.
If an investigative journalist "mysteriously vanished" in a Latin American dictatorship, for example, one would not even raise an eyebrow if told it was government work. Even if lots of people also vanished without foul play there.
It's another to make friendly with a tree at a 100 mph lower speed.
It's not speed that kills, it's the sudden deceleration at the end.