I think the simulation in Cities Skylines is also quite advanced, or not? The simulation is much more the reason why it requires powerful hardware to run on, much less the graphics.
Cities Skylines has pretty decent simulation and it uses quite a bit of raw CPU horsepower, but it only really shines with tonnes of mods (just like SimCity 4 before it).
Realistic traffic is always the bane of these simulators.
Can't speak on how demanding the simulation is/was. But on launch Cities Skylines 2 was extremely demanding in respect of its graphics due to very poor optimisation. This PC Gamer article summarises a more technical analysis, which is linked through
https://www.pcgamer.com/a-tech-analysis-of-cities-skylines-2...