Because LIDAR can detect the object at the beginning of the perception pipeline, whereas camera can only detect the object after an expensive and time consuming ML inference process. By the time the camera even knows there's an object (if it does at all) the LIDAR would have had the car hitting its brakes. When you're traveling 60 MPH, milliseconds matter.

Just to put numbers on it, 10ms at 60mph is just under a foot. I don't think that matters too much, but if we're talking 200ms that's 10-15 ft which is substantial. I have no idea how long the ML pipeline is, though.