John in NM
Sparks certainly can start fires under the right conditions, but it is pretty rare, which is why you find a lot of metal workers and even woodworkers to be pretty complacent about them.
The reason it is rare is that though the spark is hot, the actual mass of the oxidizing steel is very small, so it carries surprisingly little energy. When something burns - like wood dust on the floor - it is actually vapor coming from the solid that you see burning, basically the smoke is what ignites rather than the solid itself. This takes a fair amount of energy to start, and under most conditions grinding sparks just don't carry that much in spite of being quite hot.
Welding "sparks" (hot splatter really) is much more dangerous, as the particles are vastly bigger and carry a great deal more energy. Fires from welding are much more common than from grinding.
One should still be very aware of the potential fire danger from grinding, I don't mean to minimize that. Just explaining why so many people take it so casually.