That was my thought initially too, but I guess that if the crossing was shining all the time, drivers would simply start ignoring it after a while. The fact that the crossing lights up when pedestrians are allowed to cross acts as a visual cue for the drivers to stop.

In Switzerland we add small pearls to the paint when marking a zebra crossing. The pearls reflect the light from the cars.

http://www.tagesanzeiger.ch/panorama/vermischtes/SwarovskiPe...