You pose an interesting and logical question. When I taught classes in the Army many years ago, I allowed no "why" questions. Unfortunately this is one of those why questions that I can't answer.
I understand your puzzlement. I am very confident that what I say is true, but I'm not aware of how the heat is increased without adding air. I believe the answer relates to Jim's comment. If there is ENOUGH air for combustion, then adding more fuel to the fire makes it hotter.
A campfire has a virtually unlimited supply of air. As you add more logs to the fire, it gets bigger.
When "turning up" a diesel (as the truck drivers call it) you add more fuel to the cylinder which builds more heat and makes beaucoup smoke. I expect that once you pass the point where the incoming air is inadequate, you are also reaching a point where there is much too much fuel in the chamber. The heat is still excessive, plus you are bringing unburned fuel into the picture. Even though diesel fuel is light oil, it is still a solvent relative to the REAL lubricating oil in the engine. This means the diesel is so excessive that it is washing the oil from the cylinder walls and diluting the crankcase with "solvent".
I expect that it is this combination of excessive heat and oil dillution that cuases the quick demise of a "turned up" engine.
Hope this helps,