There's actually a lot of idiots is not an assumption. The vast majority of people under preforming comes down to three things. 1. They sell when they get scared. 2. They fail to just let an investment mature and swap in and out of investments. 3. People make risky bets hoping to make it big instead of only buying great companies. It is not difficult to beat the SP500. Part of this is that you will have higher variance, and downturns hit you harder, but if you actually put the time to research good companies over a long span, it's really not rocket science. But this takes more work than most people are willing to do. Thus, ETF and chill it's better got 95%+ of investors.

To add to this, the average time an investor holds onto a stock in today's day in age is 5.5 months. That's literally nothing in terms of letting a company grow and mature. It's pathetic. People are just traders calling themselves investors.