The way they're designed or rather how long they're designed to last assumes some very specific conditions. The biggest problem is that people thrash their cars and dont even realize they're doing it because they assume that a $20k thing should be made to endure a war zone, which they certainly are not. The way I see some people drive makes me think they hate their car, but they probably just don't realize that full acceleration all the time, not slowing for bumps, heavy braking, etc are the biggest keys to reducing the usable lifespan of their car.
Technically, anything with a bearing can fail pretty much at any time, which includes anything with a pulley - Alternator, water pump, cams and crankshaft, etc. You probably have some kind of warning signs that an issue is coming, but people either don't have time or money to diagnose every little problem or they just assume cars are supposed to make odd squeaks and squeals every once in a while.