It can be difficult to hold onto the winners and not sell them too early even as they suddenly seem expensive, you’re already up 3x or whatever, and besides there’s this shiny new IPO/turnaround/whatever where you could put the money instead...
Sticking with the index saves you from these bad decisions.
(I had some Apple stock in 2002 that I sold a year later; some Tesla in 2013 that I sold in 2016; some AMD and Nvidia that I sold in 2017; and some Shopify that I sold last year.)
To do much better than the index over a long time frame would probably require a time machine.
Proof? Warren Buffet couldn't do it. The vast majority of hedge funds, who's _job_ is to beat index funds can't do it. Of those that do, lose to index funds anyway because of fees. And these are all businesses that have several orders of magnitude advantage over an individual investor.