Perfect sensing is not assumed but since the agent has distributed control and spends most of its energy in sensing and acting (as opposed to 'reasoning' about the current state). This imperfection in sensing does not cause deterioration in an agent's behaviors.
The real world is also unpredictable. This drives the desire for low-cost, low-complexity agents which consume little power. Thus maintenance and replacement of the agents is simplified.
A dynamic world implies that the agent must be reactive if the agent is to survive within that environment.