As recently as the 1970s, this strategy would have been hard to carry out. Modern index funds didn’t exist until John C. Bogle invented the concept for Vanguard in 1976. Mutual funds in the past had much higher fees than they do today. Buying lots of different individual stocks would have required high brokerage fees as well, making it all but impossible for people with modest savings.
Moreover, the advantages of a “buy the index” approach were not as well understood until recent decades. Academic finance research in the second half of the 20th century had a series of findings about the efficiency of markets that, taken together, imply that the best long-term investing strategy for most people is simply to put money into the market as a whole and minimize fees and taxes. Personal finance advisers and commentators widely embraced this finding, with adjustments that depend on the investor’s risk preferences, particularly investing some slice of the portfolio in safer bonds.
The result: In recent decades, following the most obvious conventional wisdom of how to invest has been possible even for small investors.