Because too much of the general population thinks running is bad for the knees. My experience/friends' experiences are that advice and concern about knees generally come from average, non-active, overweight people.
My (obese) Mom had knee surgery a few years ago, and warned me about taking care of my knees and not running to much.
Not long after that, my (obese) (younger) sister needed knee surgery. At age 31.
There is a slight confounding due to new runners working on a marathon bucket list, just months off the couch, who, rather than giving the musculoskeletal system a good couple of years to adapt, keep running with knee wraps and other gimmicks as they progress through every possible injury listed in an inevitable annual Runner's World article, but these injuries should be more accurately categorized as another risk of being overweight or training improperly.
Taken together, the average experience of average people ends up being osteoarthritis and other knee problems because of being overweight. This is still the majority of patients, from which physician experience is formed. Additionally, this is also the pool from which new physicians are drawn, as there isn't a social pressure for our physicians to be physically fit.