I'm a software engineer. And yes multithreading is difficult, just slapping on async isn't necessarily going to help you run code in parallel

Think about the workload a game is using, you have to do most calcs on a frame by frame basis and you tend to want effects to apply in order. So you have a hard time running in parallel as the state for frame 1 needs to be calculated before frame 2. And within frame 1 any number of scripts can rely on the results of another, so you can't just throw threads at the problem You can do some things like the sound system but beyond that it's not trivial