The biggest problem with stock investing is

The biggest problem is that people have massive greed without putting in any damn effort..
Even with just a little knowledge of stocks, one knows that investing half in Nasdaq and half in S&P 500 yields a very high win rate and puts you in the top tier, but people get greedier and invest in individual stocks..
Investing in individual stocks isn't the problem itself.. If you want to beat market returns by investing in individual stocks, you have to put in an insane amount of effort.. At the very least, you must constantly update your knowledge on the sectors and specific stocks you invest in, and continuously think about whether to hold or stop..
But most people have plenty of greed without the effort, treating it like gambling or Baccarat.. Unlike Baccarat, however, the stock market does not give a win rate of over 50% to those who do not put in the effort.. The biggest problem is that they then go on to deny the stock market itself.