[Teknoman117]

The part that's way harder though is that at least in a game engine, you know the absolute world state and when you make a change to your world state, it happens exactly as you wanted.

In dynamic systems, such as a rocket, robot, etc., reality is fuzzy. Your loop is mostly about "what is my best guess at my present state" and "what is my desired next state". You make changes to try and get from A to B but you may not exactly achieve B. You may have not even been in state A. The error propagates back into your next guess of state, and this repeats forever.

Sensors like GPS give you an absolute position but they're imprecise. Inertial navigation is extremely accurate in telling you the change from a sample to the next, but as your position is the second integral of your acceleration, any error compounds quickly (admittedly the quality of the INS available to people willing to spend 10s of millions of dollars on a vehicle FAR exceeds what you'd be able to put in a car). Some rockets have even used ground based radar measurements uplinked to them to further improve position estimates. They probably still do this, I just don't have any hard data on current launchers.