Since the Great Inflation monetary policy has been quite intentionally stripped of money. Banks evolved and there was really no easy way to define money beyond a certain point (in the sixties), so Economists just gave up trying. This is no small thing, but in Economics it is treated trivially.
Around the same time, Positive Economics, or econometrics, came into its own. It was widely accepted as one possible answer to a lot of academic problems. Rather than being forced to study incessantly the dizzying complexity of any economic system, econometrics offered a short cut. Define a few big correlations and that was all that was required.