Humans = rational?
"Macroeconomics is a pretty shit guide to the world because we humans are still pretty shit at macroeonomics"
This hits the nail on the head, because people don't always do what is 'rationale' in any given situation. As populations, we suffer from lack of complete information about the current economic situation - not just because economic indicators are published after the fact but also because our information is filtered through a number of sources who have a vested interest (governments, non-governmental groups, the media) and so we probably couldn't act 'rationally even if we wanted to. As individuals, we also don't behave 'rationally' because we value some things more than we 'should' if we were truly trying to maximize profit or growth. Bottom line, you can't equate economics with physical sciences (such as chemistry) because you cannot ascribe rivers to human behaviour with any level of reproducibility. [Unless you are are Hari Seldon]