[anamax]

Car brakes can get hot enough to start a fire (wood, paper, maybe even gasoline - see race cars) but a spark is a hot piece of material that is large enough so it can keep its heat for a while.

Brakes generate very fine dust that cools off almost immediately.

[ColdStream]

The concentration of energy is vital. Sort of like how if you fire a gun it will only push the person back a few centimeters but that energy condensed into a very small spot and fly through a car.