It’s because there was no iteration. There was only one attempt to actually use the discovery to influence society. There should have been at least one smart bad guy who used knowledge of psychohistory to search for conditions that would favor his preferred outcomes over Seldon’s.
There’s a smarter treatment of the idea in the John C. Wright’s “Count to the Eschaton” series.
There’s a smarter treatment of the idea in the John C. Wright’s “Count to the Eschaton” series.