Because they do stupid things!
I was at an intersection today in my SUV waiting for a light. The road ends at the intersection and two lanes turn left. I was in the right lane but turning left. The light changed and the large truck to my left did not move. When I did, I found a bicyclist in front of him. The guy crossed against the light and we were all supposed to wait for him. I didn't even see him and bet the car behind me didn't either.
We cannot complain about drivers if we don't follow the rules of road, also. It takes everyone working together to keep us safe on the roads!