100 percent can relate on the relentless curiosity of a child. I was dumbfounded yesterday by this question -> if cars go at high speed, and breaks stop it suddenly, how come the friction doesn't generate enough heat to start a spark?
It's just that the energy is spread out over a larger area (brake discs/pads as opposed to a tiny spark) that is also constantly being cooled by air movement, so the energy in any one point is not enough to ignite any of the materials involved. It's why they used to put asbestos in the brake pads, but they now use ceramics and other heat-resistant materials.
Also, engineering.
For a visual spark, it has to be something thrown off which is the right size, material, and a temperature to glow visibly to the human eye under normal light.
For a spark that starts a fire, it has to be something thrown off which has a certain temperature by the time it lands or travels to some other surface.
In both cases, hot very fine particles will cool quickly just by hitting the ambient air. If the underlying surface (e.g. brake disk) gets glowy-hot, we wouldn't call that a spark--it's too big, and fixed.
Oof, the guy who went into the definition of sparks actually makes me realize this question branches off into so many different rabbitholes.
In my experience sparks are generally generated when quite small metal surfaces get in contact at high speeds, meaning they can generate a lot of friction in a very small point, and thus not have enough braking effect despite the local friction in one small point (think rail wheels squeeking in curves).
Brakes OTOH are designed to spread out the friction on a somewhat larger area, which allows more material to take up the friction and the heat, and spread it around.
I know brakes can get extremely hot too though, and after a really hard braking you might have to worry about deformed brakes sometimes.
Race cars tend to have really large breaks (and brake pads) for this reason.
https://www.youtube.com/watch?v=CUnEeiZK7bY
And, if you let the pads wear down too far so they're metal on metal they absolutely will spark.
How cool are those rally cars though?
Brakes generate very fine dust that cools off almost immediately.
When a car slams on the brakes the distance is too short to generate enough heat to deform. I would also suspect the treads would partially melt and deform before it catches on fire.
Only kidding. Kinda.