Why aren't runways lit from overhead, like highways?
Because the planes might hit them and it's not great for your vision at night. Runway edge lights are not intended to illuminate the runway they are intended to outline it. Proper runway lighting very clearly outlines a runway at night 


(source)

Generally speaking there is nothing saying that a planes wingspan needs to be narrower than the runway and at some smaller airports the wings may very well be wider than the runway. This means you would need to put any high mast lights far enough away to allow aircraft clearance. In turn they would need to be quite bright due to the distance. 

Another key reason and perhaps more important is to preserve the pilots "night vision". Generally when flying at night its advised to not expose your eyes to any bright lights prior to flying and to ensure that no flash lights or other bright lights go off in the cockpit while flying. This can cause your pupils to constrict making it harder to see in a dark environment. Entering a very bright runway environment all of a sudden would be strenuous on your eyes, in the event of a go around you would then have night vision issues due to the exposure in the brightly lit runway environment. As per the FAA's suggestion:


  Before and during night flight, the adaptation process of the eyes
  must be considered. First, adapt to the low level of light and then
  stay adapted. After the eyes are adapted to the darkness, avoid
  exposing them for more than one second to any bright white light as
  that causes temporary blindness. If exposed to a bright light source,
  such as search lights and landing lights, remember that each eye
  adapts to the dark independently. By closing or covering one eye when
  exposed to light, some night vision acuity is retained in the closed
  eye.