Index fund tracking has become successful because one of the goals of US Government policy is to maintain that things like the S&P 500 continue to go up over time. If they go down people get voted out of office. So the index funds have the full force of the US Government watching over them. This is why economics is not just some math puzzle but often more about politics and sociology.
The prices of stocks are one of the most forward-looking macroeconomic measures that we have available to us. The better we get at improving long-term economic growth, the higher stock prices will go, other things being equal. So basically, if you have good policy, stock prices should always go up (in real terms). If the Federal Reserve were perfect at preventing recessions and everyone knew it, stock prices would be higher.
While you would probably not want to make stock index growth the target of monetary policy because of Goodhart's Law, it is very reasonable to take it into account as a part of a forecast of how well your policy is working.