I see simulation as an attempt to create a predictive model of a setting. This leads to some other assumptions:
In the absence of information to the contrary, we should expect that things are meant to seem as realistic as possible. A character that doesn't get enough sleep or food will not be as aware or active as they could be (to use two simple examples).
Complexity - or at least, more rules modules - is often used to bolster the simulative process. If I want my assassin to take a shot in the dark, I need night vision capability. Do I then need rules for encumbrance, or weather, or degrees of darkness, or how wearing the equipment reduces my peripheral vision?
The more unusual the setting, the harder it is to simulate, because people can have different conclusions about how the setting would shake out. If elementals pull trains and heat houses, why is there still scarcity in the economy? Why don't droids rebel?