Well, with a bare fist you are far more likely to break your hand on someone else’s face before you break theirs. Fiberglass gloves, brass knuckles, those change the equation, but the bones in your hand are very fragile compared to the ones in the rest of your body. Those much more hardy bones are the ones the punch is going after. It’s not a matter of just giving, you receive a portion of your own expended force back up your arm. It’s very difficult to break someone’s bones with a bare fist. Bruise or stun them, sure. Break them? Not so much; it’s better to go after soft targets like the stomach, the throat, or the jaw, and use a palm strike or a hammer fist that are more reinforced.
Not only that, but your major bones are designed to take a large amount of abuse. Breaking them with a crowbar or a sledgehammar is easy, but that’s using a weapon can bring a great deal more force than your fist can generate. With your basic body you need to apply pressure specifically along an avenue that the bone isn’t meant to take. I broke my leg doing martial arts when I was twelve the first time I learned to do a tornado kick and it wasn’t the kicking leg which broke. It was the supporting leg, the foot got stuck on the floor, my body kept rotating and my tibia snapped from the built up pressure.
It’s just not worth the energy expended. If you want to break bones with your hands, do CPR.
Kicks are a different story. Kicks generate enough force to crack bones, even if they don’t outright break them. This is mostly me focusing on the ribcage, where it certainly does happen. A spin kick like a wheel kick can generate enough force to sever the spinal column with a kick to the head if you get lucky, which is a big if. The brain can be damaged by knocking it against the skull, even resulting in death. Again though, kicking someone in the head with no protections (or even with protections) is always flirting with brain damage and even death. You can’t really control whether or not it’s going to happen though, some fine muscle control does go out the window when at the mercy of the momentum of a spin. (High line kicks are very chancy outside controlled environments anyway.) What I’m saying is that it isn’t 100%.
The end result is this: your bones are actually very difficult to intentionally break. It’s better to go after other soft targets on the body like the stomach, the kidneys, the throat, the eyes which will achieve a better result. When you go after bones, like the shin it’s because exposed bones are sensitive and unprotected by a layer of muscle/fat. They have a lot nerves which when hit cause pain that distracts an opponent.