Because many people don't. The sheer amount of cyclist that don't respect red lights and run through pedestrians crossing demonstrate this. I could film a busy crossing where I live and catch an infringement in less than 5 minutes.

My wife, while pregnant, got hit by a cyclist while crossing in Paris. She was well aware of what she was doing, had a very expensive bike and surely was riding everyday to work. But heh, can't slow down to let the peasants cross the road, right?