Sealed cartridge bearings aren't 100% waterproof. If you submerge them regularly or if you spray high pressure spray at them, water will get in and contiminate the inside of the bearing. The reason why automotive bearings last for so long is because of the design and how they are positioned inside the actual wheel hub. The hub and various seals and shields, protect the bearing from dirt and water intrusion under normal driving conditions. What will shorten the lifespan of automotive bearings is going off road and driving through deep water and deep mud frequently....The bearing used on bicycle bottom brackets and hubs are not really protected very well and if you point a high pressure spray at them water will get inside. That's why full fenders can extend the life of bottom bracket and headset bearing because they prevent road spray from going directly sprayed at the bearings. The fender acts as a shield.