If everyone's strategy were to buy some collection of assets and sit on them, you would expect some definition of the average individual to earn the average market return. But users of trading platforms are probably trading actively, and making mistakes. It's also likely that the average user would benefit from a diversified portfolio to reduce the risk of losing big all at once. They're less likely to get that picking stocks themselves.
The average professional fund manager actually underperforms the S&P. It wouldn't be unreasonable to assume that the average retail investor does even worse.
However, there's an easy way for people to overcome this - buying an index fund (Vanguard is very cheap), which lets you capture (almost) the S&P's performance.
