What would make you think that? Deterministic doesn't imply predictable, reducible, or low complexity. Even a purely deterministic simple system can be capable of (and most are) chaotic behavior whose information content would grow faster than any system smaller than itself could predict. In other words, the only way to "simulate" the system would be to completely reproduce every bit of it, and let it evolve in realtime. And practically speaking, we can't even approach that.