What makes atrial fibrillation not immediately life threatening while ventricular fibrillation is?

A circuit breaker. The rhythms in the top chamber during atrial fibrillation can reach and exceed 600 bpm. If this were conducted directly to the lower chamber, it would be as fatal as ventricular fibrillation. However, nature built in a circuit breaker called the av node between the two such that only some of the top chamber heartbeats in fibrillation make it to the bottom chamber.
AV node . Afib has the protection of the av node which is electrical relay station between the artia and ventricles which slows conduction and limits rate of signals to the ventricles. With vfib no protection exists.