[My_Name]

They generate enough energy to, I mean have you seen racing brake discs?

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.

[Terr_]

I'd approach it by looking at the definition of a "spark", and the things we can change to avoid tripping that definition.

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.