Psychohistory

Why doesn't Asimov's Foundation psychohistory work in the real world?

Maybe it does but we don't want to know the answer. Malthus. Every empire inevitably crumbles, and the end is never pretty. We don't want to learn that, over the years, we haven't gotten any "better", for a suitable definition of "better".

Maybe it does, but the answers it gives us are boring. Population will continue to increase, as will productivity.

Maybe it doesn't work because:

Technology causes variance to increase over time, so the model has little predictive power.

We don't keep good enough records, especially suffering from the fact that history is written by the victors.