What you’re seeing is the nature of the beast:
A simulator like this will always have a ‘main thread’, that keeps all the tasks running on the other threads in sync. This main thread can only run on a single thread (or in your case, since you’re not using HT, a single core).
There will be small optimizations in the future, moving more workloads away from the main thread, towards other threads, but those new threads will need to be kept in sync as well, so it will still have a load on the main thread.
Most games (and complex simulators especially) all have this characteristic. More productivity focused software (like tile based renderers) will be able to utilize all the cores/threads more effectively, since in software like that it doesn’t matter that 1 task is done earlier than the others. In a simulator this simply can’t be done.