I believe that anatomically the hips and legs are best used forward and backward (as in walking) and less so in sideways movement.
So exaggerated movements like high kicks are less potentially damaging for front kicks and back kicks, and more potentially damaging for kicks that raise the leg to the side, such as side kicks, roundhouse kicks, and hook kicks.
Of course your legs can move to the side; the human body has an amazing range of movement. But if the movement is exaggerated, as is done with high kicks, and particularly with kicks that raise to the side, and done with high repetition, over many years (just how many will vary from person-to-person) the hips can become damaged. It’s like any sports related injury that comes from heavy use and repeated injury. Football players can end up with joint injuries from the rough nature of the game. Competitive swimmers can develop shoulder injuries from years of heavy training. Martial artists can end up with repetitive motion injuries, especially if the movement is extreme.
I’m not a TKD person but I had clean, high kicks in my training, for years. My training has had some interruptions in the last few years and I have spent far less time with kicks than I did in the past. I’m also getting past my physical prime, as I turned 49 earlier this year. My front kicks are still clean and I can easily do them high. My back kicks are clean, but I’ve never done them above the midsection. But my side kicks, roundhouse kicks, and hook kicks really deteriorated. I’ve lowered them dramatically and it has taken time to clean them up again. I anticipate never using them high anymore, and I’ve decided I’m not a big fan of the hook and roundhouse anymore. I think that speaks to the natural movement of front-to-back, and less natural for raising to the side.
I had a teacher a number of years ago who grew up training in the 1960s and 1970s. That was an era when people used little or no safety equipment and would beat the crap out of each other in sparring. He had his hips replaced, and I believe that early training with heavy contact contributed to it. People like to glamorize the “good old days” when people trained hard and “really learned how to fight” and contrast that with the “snowflakes” of today who can’t handle it. I think that’s nonsense. There needs to be a reasonable balance between solid training and safety. But stupid training is stupid training, and if you beat the crap out of each other in your youth to spend the last 30 years of your life barely able to walk, I would say that was stupid training and not a good trade. The problem is that you often don’t realize you are doing the damage until it is done and cannot be avoided. So you need to have some foresight and make some smart decisions about how you train. And you need to recognize when someone like an instructor is demanding that you do something in your training that you recognize is damaging to you, even if not immediately damaging but will be in the future if continued. And you need to be ready to say no. And some people will be able to get away with things for their whole life, without injury, that other people won’t. So people need to make personal decisions and recognize that there will be contrary examples to be found.
So anyways, there can be more than one factor that goes into this.