Multiple scales of complexity and interaction, that’s way too computationally intensive to just directly stimulate how reality works, and so each scale has to be carefully “averaged out”, in a sense, to end up with a simulation that is cheap enough to run yet still behaves realistically and reproduces as many nuances as possible.
Your last paragraph reminds me of the following series of YouTube videos that goes over how to build a “correct” fluid simulation, starting at the quantum mechanical level (iirc): https://www.youtube.com/playlist?list=PLMoTR49uj6ld32zLVWmcGXaW7w2ey7Vh4.
Multiple scales of complexity and interaction, that’s way too computationally intensive to just directly stimulate how reality works, and so each scale has to be carefully “averaged out”, in a sense, to end up with a simulation that is cheap enough to run yet still behaves realistically and reproduces as many nuances as possible.