I'm a spooky ghost! Whooooo!!! Ectoplasm!!!!
It is over a certain weight. The force pn your feet and knees will feck them right on upOther than it also cover generalised knee pain, and that osteoarthritis is the 'wearing out your knees' that everyone seems so worried about...
I don't trivialise the need to get stuff right. If you're in pain, stop, build slowly, and never try to do too much too soon. But it's simply inaccurate to suggest that running is inherently unhealthy or bad for you.