It was once thought that the money supply was directly related to inflation, but they were never the same thing. The classic equation was MV = PY, and M (the money supply) and P (the price level) are different. They are only proportional (according to this equation) if everything else remains the same.

In any case, the "money supply" is an abstract macroeconomic variable with multiple possible definitions. Why do we care about it? Because it might have a real-world effect on us via price changes.

Gathering data about prices directly is a better way of understanding price levels (and inflation) than mucking around with less measurable quantities.

To the extent that the money supply matters, it's because it might result in higher prices in the future. But this doesn't seem to happen in any mechanical way. Just because people have money doesn't mean they want to spend it. In the classic equation, V (the velocity of money) can slow down.

This is particularly true when we are talking about institutions and rich people who already have savings. Higher numbers in their bank accounts doesn't automatically result in more spending, either by them, by the banks, or by companies whose stock prices get bid up.

It would matter more if the money went to people who actually need to spend it.