Because that's the BASIC DEFINITION of simulation. If the game rules do not tell you why something happens, then they are not simulating ANYTHING.
So, no, D&D absolutely does not simulate a heroic fictional world. It really doesn't. You have added all of the simulation yourself. At no point is the system simulating anything. The fact that you straight up say, "why should the game rules tell us why or how" something happens means you actually don't know what the word simulation means.
This system is only a simulation because you like it and you think that you like simulations. But, it's pretty clear here, you don't actually know what the word means. A simulation that does not tell you how or why is not a simulation.