> Right, but the simulation isn't as complex as simulating rain in a 3D game or whatever

I think a naive implementation of factorio would be reasonably straightforward. But that implementation would have no chance to scale to the size of bases factorio manages. The complexity of factorio’s implementation comes from its simulation’s optimizations, particularly in the face of its hard determinism requirement. (Something most physics engines and rain shaders never need to think about.)

Making a good rain shader takes expertise but probably not a lot of time when you know what you’re doing (months not years). Making a game like factorio probably doesn’t take more specialised expertise than many devs here have, but even with all the requisite knowledge it would still take me years to implement something as feature rich, correct and performant as factorio’s simulator.