Most people who pick individual stocks end up behind a plain index fund. This is not a slur on their intelligence — the market is full of extremely clever people, and that is precisely the problem.
The arithmetic that cannot be argued with
All investors collectively own the whole market, so collectively they earn the market return before costs. That is an accounting identity, not a theory. After costs, the average active investor must therefore earn less than the market by exactly the amount they pay in fees, spreads and taxes. For every active investor beating the index, another is losing by the same amount — and both are paying for the privilege.
Returns are brutally skewed
Market returns do not come from the average stock; they come from a small minority of enormous winners. The median stock does considerably worse than the index, because the index's return is dragged upward by a handful of extraordinary performers.
The implication is uncomfortable: a portfolio of ten stocks chosen at random will most likely underperform, not because you chose badly but because you probably missed the few that mattered. An index fund owns the winners automatically — it does not need to identify them.
You are trading against professionals
When you buy a stock because it looks cheap, someone is selling it to you. That someone is often an institution with analysts, data and a direct line to management. Your edge has to come from somewhere, and "I read the news and it seems undervalued" is not an edge.
The behaviour gap
Even investors who pick reasonable stocks tend to hurt themselves with timing: buying after a run, selling in a panic. Concentrated portfolios amplify this, because a single position falling 40% provokes far more action than a diversified fund doing the same.
Does this mean never pick stocks?
No — but be clear about why you are doing it. If it is for enjoyment and education, cap it: keep the bulk of your money in an index fund and run a small satellite portfolio you can afford to be wrong with. If it is because you believe you can beat professionals, be ready to measure yourself honestly against the index every year, including the years you would rather not look.
Summary
The index is not a mediocre outcome — it is the return of the collective, and it beats most of the people trying to beat it. That is the whole argument for passive investing.