Originally Posted by JoeyO
How come LED bulbs are made to replace fog light halogen bulbs and every other bulb on the car but they're not designed for the regular driving headlamp halogen bulbs? In other words, the lights that really matter.
AFAIK, it's because you need roughly point light source at the focal point of the reflector to create the beam for the main lights and that takes special high-power LEDs or maybe some projector system.
Notice that the LED replacements have multiple small LEDs mounted on a cylinder to generate ~same total light output as the halogen. This is fine if you just need light that gets scattered in multiple directions for turn signals, brake lights, etc., but not so good if you need to create a beam of light.