As Steve explained, it's even worse than that…the standard economic models have the assumption of not just a perfectly rational set of actors, but every one of them endowed with perfect clairvoyance…able to integrate all current knowledge into a framework that allows them to make perfect decisions.
This was needed because otherwise the models just didn't behave properly.
Which is why you have jokes like this one; three economists are locked in a basement and are beginning to get hungry, but they are not worried. Why Not? Because soon enough one of them will just assume sandwiches to have always existed in the basement.
The real mystery is that despite the massive failings of standard economics over the past decades at explaining or predicting anything at all useful, their every word is still fawned over by the world…