Why is 'lane splitting' (filtering between lanes of moving motor vehicles) hazardous on high-speed roads?

1. Cars may change lanes abruptly without signaling, car doors may open, and the tight lateral clearance leaves zero escape space for the rider
2. It is safe as long as the rider travels at the same speed as surrounding traffic
3. Drivers can always see a motorcycle passing between lanes in their mirrors
4. The practice presents no more risk than ordinary overtaking on the left