Beating the S&P 500 used to be the goal for active investors — now it's more like chasing a mirage.In a note shared this week, Torsten Slok, chief economist at Apollo Global Management, delivered the hard truth: "There is no alpha left in public markets." And the data seems to back him up.In a landscape increasingly shaped by aging IPOs, dominant mega-cap stocks and the rise of passive investing, active investors are finding fewer ways to generate excess returns. The idea of "alpha" — the holy grail of beat ...