Sheldon Richman offers his insights on that question in this Reason essay.
Cato’s Daniel Mitchell says that Keynesian economic theory is the equivalent of the magical weight-loss pill. Such pills keep selling even though many people know that there is no miracle cure for being overweight. Similarly with the economy — there is no miracle cure for the damage that the governmentally-induced boom and bust cycle does. It takes time for all the economic actors to readjust, just as it takes time to lose excess pounds.