Strong doesn't equal rigid. Think of the crank as a very strong spring (or torsion rod if you prefer). Every time a cylinder fires, it winds the spring up a little. Then the spring unwinds a little. The twist in the spring causes the journals to angle a little bit, closing the gap (formed by the oil film) between them and the bearings. Bearings and journal touch, get hot, Babbit (bearing material) gets hot, melts, bearing friction increases, bearing spins, engine knocks, oops, new bearing and crank time.
