Because whether it's people pulling the trigger or the gun itself, both are the direct source of harm. Bicycles are not the source of harm, cars are.

The analogous argument one way would be quibbling over whether it's the driver or the car. The analogous argument the other would be saying it's the shooting victims fault for being in a place where they are likely to get shot (bad neighborhood, in a position to surprise or threaten an armed individual, or maybe just in America)