Cracking one's knuckles has been a thing that men do, that is: only a minority of women would be engaging in this, but men would pick it up in groups and it would become a custom. And of course, boys at a younger age would emulate the elder men, and so you had boys cracking their knuckles, and doing it at home, in front of their sisters and their mothers, and doing it in school in front of their teachers. And it is a blue-collar activity; something considered uncouth for the genteel knowledge-worker to do, except if you're acting in solidarity with the manual laborer, and you crack your knuckles as you sit down at the keyboard to type.
So the cracking of knuckles annoys people who don't do it, and it annoys women, and it annoys people in authority over young boys. So you want them to stop, and you want to regulate men's bodies and control what we do with our bodies. But you cannot tell us all the time "because I said so" or God forbid, "because it annoys me", because that can eventually achieve the opposite result.
So the medical myth of developing arthrosis and hurting your bones and damaging your joints, those were "old wives' tales" and "white lies" that were told in order to scare boys into stopping the activity. And perhaps even to scare husbands and employees into stopping. They tell men to stop spitting on the floor, to stop picking their nails with a pocket knife, and usually a medical excuse arises, because this is a higher authority that will terminate any individual choice or discretion in the way we treat our bodies.