The book, first of all, does a good job of laying out the complete case for why index funds beat human managers, starting out by telling the story of an investor who made that very bet (literally), who ironically most people would put on the other side of that: Warren Buffett. Buffett bet a fund manager that an index fund would beat the best of any funds his opponent would select. A decade or so later, Buffett was proved right and declared the winner.
https://www.advisorperspectives.com/articles/2017/11/13/ted-....
If even Warren Buffett is betting on index funds as beating active investors, why are most people so reluctant to?
But besides that... why are index funds - which you can also think of as 'fully automated/computerized investing' - so effective?
There's a few reasons, but a really simple explanation is: over time, a bunch of fund managers are just going to recapitulate the market, and the advantage computers have is, they can duplicate that functionality but their fees are zero (or 0.001% per year, whatever is the tiny amount it costs to run the servers). Human fees are not zero. Therefore, the computers win.
Another one is just that the whole market, diversified, ends up being the best hedge against... everything. Is Elon Musk a genius one day, and now getting pilloried as an idiot as the head of Twitter? If he's a big part of your portfolio, and you bought in a year ago, that really hurts. When you hold the whole market following impersonal computer rules, you are insulated from this.
And it turns out, when you look at the full impartial record of active investor picks, it's pretty poor really.
A final thought that book suggested to me: if you have over 10 million dollars, you probably can't beat what the market can do. Wave the white flag and throw it all in an index fund. The computers have won, and just as we acknowledge that with chess and Go (art's probably next), index funds are an extension of same.