Another problem with bike headlights is that they are physically very small sources. They're made to be as small as possible for weight and air resistance reasons on a bike. The smaller size of the image of them on a viewer's retinas makes the energy density of the image higher for a given amount of light entering the eye. Compare the area of a car's headlight with the area of a bicycle's headlight. Compare a motorcycle's headlight with a bicycle's headlight. For equal amounts of light going through your iris, the bicycle headlight will be waaaaay brighter.