If the S&P 500 is such a reliable long-term investment, why don't banks offer 20-year investment loans?

I've been thinking about this and I'm wondering what I'm missing. Let's say someone applies for a $100,000 loan. Instead of giving them cash, the bank invests the money directly into an S&P 500 index fund and locks it up for 20 years so the borrower can't withdraw it or misuse it. At the end of the 20 years, the bank gets its original $100,000 back plus some agreed-upon share of the profits. Historically, the S&P 500 has returned around 10% annually over long periods. So $100,000 could potential