No, they're not. Other people stress their body just as much as Nadal and are entirely healthy in the knee department. It's just bad luck. With something as complicated as the human body, a simple 1+1=2 won't do, DJ.
No, other people don't do it as much as Nadal. And no, it's not bad luck. People has being saying Nadal's gonna grind his bones for real since, what, 2005? This is not bad luck. This is not a freak accident. This is something very predictable and unsurprising.
What gives Nadal his success, also gives him grinded knees.