> What is the critical difference?

  * Observers are also actors. They can and will significantly influence the observed.
  * Once observations are revealed at large it'll have short/long term consequences. There's a good reason fed gives multiple indications before changing any parameters such as interest rates, tapering etc.,
  * Humans don't necessarily act like rational selfish agents (as economists expect) all the time.
  * Economics is often entangled with other aspects such as trade agreements, geopolitics some of which take years and decades to manifest.