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This is more of a mathematical axiom than a financial effect, because you're defining "underperform/overperform" with respect to an average that contains them.Most stocks suck:
> We study long-run shareholder outcomes for over 64,000 global common stocks during the January 1990 to December 2020 period. We document that the majority, 55.2% of U.S. stocks and 57.4% of non-U.S. stocks, underperform one-month U.S. Treasury bills in terms of compound returns over the full sample. Focusing on aggregate shareholder outcomes, we find that the top-performing 2.4% of firms account for all of the $US 75.7 trillion in net global stock market wealth creation from 1990 to December 2020. Outside the US, 1.41% of firms account for the $US 30.7 trillion in net wealth creation.
* https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3710251
> Four out of every seven common stocks that have appeared in the CRSP database since 1926 have lifetime buy-and-hold returns less than one-month Treasuries. When stated in terms of lifetime dollar wealth creation, the best-performing four percent of listed companies explain the net gain for the entire U.S. stock market since 1926, as other stocks collectively matched Treasury bills. These results highlight the important role of positive skewness in the distribution of individual stock returns, attributable both to skewness in monthly returns and to the effects of compounding. The results help to explain why poorly-diversified active strategies most often underperform market averages.
* https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2900447
And this certainly can have a financial effect on your finances: having the "wrong" stocks in your portfolio (i.e., most of them) and not have the "correct" ones will mean a (e.g.) comfortable retirement or not.