Its a two pronged problem. First there are just too many elements with a lot of complex dynamics at the molecular level. Our hardware is just not good at that type of task, especially when you scale it to trillions of cells, and then the environment around them at the molecular level because that is a huge factor as well.

The second problem is that we also do not even know all of the dynamics, so we don't exactly have all the data necessary for running the simulation in the first place. We don't have a full account of all the metabolic and signal transduction pathways and various other processes, and how they intersect each other. We can't exactly get a real-time view into a living organisms cells at molecular resolution.