The problem with modelling each investor as an agent is that we run into other problems.

Some investors will buy a stock when it goes up, making the stock go up even higher. This is an example whereby an agent produces positive feedback loops. Other investors will do the opposite and buy the stock when it goes down. This is an example whereby an agent produces negative feedback loops.

A system with primarily positive feedback would be like cryptocurrency markets when prices of bitcoin blew up or collapsed. If you really believe that the cryptocurrency markets will moon over the