Simulation is difficult because often the thing being simulated is a piece of hardware that doesn't exist yet and is more advanced than what is available today and so simulating it can require significantly more resources than running the actual hardware would.
@xyz, agreed. Simulating can potentially consume a lot of resources, particularly if we try to simulate the full machine. However, as the slide mentions, if the machine is sufficiently scaled down, and it is ensured that the simulation mimics the actual hardware fairly accurately, then the simulation can be extremely important to conclude whether the proposed machine will achieve its goals or not. It kind of helps me imagine how companies finalize on a design of a processor after considering thousands of different permutations and combinations of the many components of these machines.