I don't see any fundamental difference between what your call simulation and what economists call models. The only difference is that your believe in the possibility of models so accurate that one could easily verify that they give correct predictions, as opposed to existing models which are subject to continual debate.

The reason it is so hard to stimulate the economy is that it represents the sum total of a large part of all human activity. So for example to test the impact of a particular kind of fiscal stimulus, you would have to accurately model the response of individuals to their new circumstances, as well as the response of markets (which involve individuals with conscious knowledge of the new policy, and thus their own beliefs about its effects). It is this last factor that makes the economy so hard to stimulate. You are not only simulating actions, but beliefs, and beliefs about beliefs, etc. It's ironic that economics is criticised for following physics to closely, when it is precisely this last factor that makes economics fundamentally unlike physics. The closest thing to modern macro if mean field theory.