"Why not?", and "Why Not?"

"Why don't alternative braking methods illuminate brake lights?"

The reason is quite simple... The truck driver doesn't step on the brake pedal.

"Why don't they step on the brake pedal?"

When professional truck drivers slow down, they avoid stepping on the brake pedal because this causes the brake pads to contact the brake drums (or rotors, if disc brakes). This causes friction, which slows the truck down. Unfortunately, this friction also causes heat, which breaks down pads, drums and rotors, wearing down expensive parts which must be replaced. Since our cars and light trucks are much lighter than heavy trucks, they do not wear out brake parts as quickly as heavy trucks, and since the brake parts on heavy trucks are much larger, they cost much more to replace. On top of the cost of parts, a truck only makes money when it is rolling, so the down time required to replace brake parts only adds injury to insult. Besides, engine braking and downshifting can slow a heavy truck much faster than using the brakes.