Difference in the Powers of Simulator and Environment in Universal Composability
In the real world we do have adversary and environment. The environment interacts with the protocol (real function) and the adversary. The parties are part of the environment. The real function leaks information to the environment/adversary
However, in the ideal world there is no such leakage. Therefore, in order to prevent the environment from distinguishing between these two worlds we need a simulator to create these leakage by getting some information from the ideal world. Here, the simulator is acting like the adversary for the real world. The environment thus is able to interact only with the ideal functionality and the simulator where the simulator is the adversary for the ideal world