Why aren't bikes required to use lights at all times?

Bikes are harder to see than motorcycles, which according to the NC DMV website have their headlights on all the time because the headlight comes on automatically when the engine is running.  Bikes are much harder to see than cars, which have lights on at night, unlike most bikes I see.  (Since most bikes I see at night don’t have lights I didn’t even know it was a law for bikers to use lights at night until someone on this site told me recently.)  And the consequences of not seeing a bike are much more severe.  Considering all that, and since bikers are both allowed and encouraged to use the same roads as cars and motorcycles, why aren’t bikes required to use lights at all times?