This is simply not correct. City Skylines 2 even went through the trouble of using DOTs which is something you cannot take advantage of in Unreal Engine or Godot. To get more optimal than that on the CPU utilization side, you will be writing your own engine in C++ or Rust.

The fuck up here is whoever was handling the art assets. You simply do not ship a game with such detailed graphics and no LODs. They must've simply been downloading things off the asset store and throwing them in without any regard for performance.