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.
https://www.pcgamer.com/a-tech-analysis-of-cities-skylines-2...
But yes, at launch Cities Skylines 2 was very heavily GPU bound, due to very unoptimized meshes and a poor culling implementation. I haven't profiled it afterwards, but from what I've read they've optimized it enough that on most systems the limiting factor is now the CPU.
Realistic traffic is always the bane of these simulators.