That's not true anymore. Most of the AI "thinking" can be multithreaded, physics can be multithreaded, rendering can be multithreaded, even core systems like loading of ressources is heavily multithreaded. It's not easy, but it's a reality for most gamedev now. However, there is a limit of how much can be multithreaded and how well it'll scale. There's a lot of inter dependencies between objects and systems that force some level of serialization, just like any other multithreaded application.