The reason is that the basic paradigm in economic theory is one in which individuals take decisions in isolation using only the information received through some general market signals, such as prices, to make their decisions. The standard model does not deny that agents interact but, as Samuelson said, they only do so through the price system. Yet agents do, in fact, trade with each other, communicate with each other and learn from each other. There is of course an approach, the game theoretic one, which takes direct account of this. However this is at the opposite extreme.
Every player takes account of what every other player does and moreover knows that the others do so.
This leads to computational problems which mean that only limited examples can be fully analysed and also to basic logical problems. My position is that we need an approach which lies in between the standard model and the full blown game theoretic model but which allows for various forms of interaction. Perhaps the easiest description of this view is to think of the economy as a complex system where aggregate behaviour is determined by the complicated interaction between individuals at the micro level. The analogies with physical, chemical and biological systems are obvious. It is not however to develop a methodological position along these lines.