Of course the IMF knows that its policies do not deliver the results they promise; the promised results have nothing to do with the real goal of the policies. The real goal of the policies is transparent, if you simply look at the inevitable results they are designed to achieve. You do not lend a bankrupt country billions of dollars on the condition that they implement reforms which will decrease the government’s revenues because you actually expect to get your money back. You lend them the money to enslave them in debt; to subordinate and marginalize the government, and to force it to liquidate its assets by selling them off to private investors. The IMF is not a lender, it is a broker for the buying of nations.
Of course, those in power know that there are alternative economic policies which will actually serve the good of the populations and deliver healthy, widespread economic growth. They have come up with neoliberalism specifically as a remedy to those policies, because those policies are democratizing in nature. They would uplift the condition of the general public; they would lead to a better educated and more politically aware and active population. This is something they do not want to happen, for the obvious reason that it would present an unmanageable challenge to their monopoly on power.