Cars that move significantly faster than other traffic (police cars, ambulances) and cars that move significantly slower than other traffic (heavy equipment, delivery trucks, mail carriers, etc.) definitely do use flashing lights. Other than emergency vehicles, though, they flash lights with a diffuse beam, not a projecting beam, while their headlights remain steady.

Blinking/flashing headlights are generally illegal on motor vehicles other than emergency vehicles, because flashing a bright projecting beam is distracting, disorienting, and dangerous in traffic.

Many motorcycles, however, do have "modulated" headlight intensity -- they vary the intensity up and down by a small percentage, enough to increase conspicuity without the drawbacks of a flashing light. It was an effect that came naturally when motorcycles had magnetos; now that motorcycles have alternators and larger batteries, the modulation is added intentionally.