Rich people have one thing in common (when they earned it themselves) they have a high drive for success. They want more! So 15-20% compared to a new business venture which could pay off big time in a few years. Also, mutual funds are not 100% save. Theres a company or bank behind a mutual fund which will buy stocks, ETFs, futures etc. with a pool of money from the customers. The company or bank of course takes a big chunk of the profits and if you Believe it or not they make mistakes too. You can't image how much money was lost in 2008, there were dozens of big investment firms going bankrupt. Also many of these firms do shady business. They always try to invent new instruments which are as complicated as possible to understand. You should watch the movie "the big short", that will open your eyes.
Why would you give someone your hard earned money to "gamble" around with it if you could just do it yourself? Especially nowadays it's easy to make a portfolio by yourself with fairly "save" ETFs or big company stocks. I remember a story I've read about a trader of a bit investment company in Japan. There's all sorts of different trading or investment styles, but this company was doing hft futures trading. Meaning they buy and sell future contracts worth billions every day. One day this particular trader came completely drunk to work, pushed some wrong buttons and shorted a futures contract instead of buying it. They lost millions or billions from that single mistake.