The only thing physical about Factorio are vehicles and particle effects. That's not where the difficulty comes from. It's more about optimisation: their entity system minimises memory use (and caches misses), many things are special cased for speed…
Think about the sheer scale of a 10 rocket per minute megabase. The amount of inserters, robots, belts, factories, the pollution involved, the number of biters attacking the base constantly… The number of entities on such a base is enormous, yet they still manage to compute it all in real time at 60 ticks per seconds. That feat of engineering is just as impressive as a 3D physics engine — perhaps even a 3D rendering engine.
(Of course, a AAA game is more than just a physics/rendering engine.)
Think about the sheer scale of a 10 rocket per minute megabase. The amount of inserters, robots, belts, factories, the pollution involved, the number of biters attacking the base constantly… The number of entities on such a base is enormous, yet they still manage to compute it all in real time at 60 ticks per seconds. That feat of engineering is just as impressive as a 3D physics engine — perhaps even a 3D rendering engine.
(Of course, a AAA game is more than just a physics/rendering engine.)