It should be pretty straightforward why the agent based models appear to be very simple. They do poses technical simplicity, which my cause some doubts on the actual usefulness of the agent based models. But the fact is that they are able to reproduce complex, non-trivial behavioral patterns.
There numerous examples for this. One of the most simple, yet very persuasive, is made available on the Icosystems website (see http://www.icosystem.com/demos/thegame.htm). This model by Icosystems discribes a very simple game, which can be played with living people. The main point of the game is to stay in certain position relative to two other randomly selected agents (or people). In one case, in the defender-aggressor scenario, the agent tries to stay behind one of the selected agent (defender) to be protected from the other (aggressor). In another case, the defender-defender scenario, both randomly selected agent are assumed to be defending the agent from the external influence, thus he tries to stay between them. Neither of agents are cooperating – they just move to take their own positions to fulfill their scenario conditions. As you can see in Fig. 1 the small difference in the game rules causes essential changes in the externally observed behavior.