Because of the general overall attitude of arrogance on the roads.
There are a few responsible bikers, but the idiots who do things like cut between traffic to pass people, and by that I mean riding on the dashed line in the middle of the road on the INTERSTATE at 70 MPH, sure do a lot of damage.
The *Watch out for motorcycle* signs need to be directed towards motorcyclists to stop taking insane chances against a car.
That particular maneuver is known as lane-splitting, and it is perfectly legal in California.
“Riding on the dashed line...” is legal in some states.