If investing in the S&P 500 is such a surefire way to make money, then why isn't everyone doing it?
First, lots and LOTS of people (and companies do it).
Three reasons people don't do it:
Some people believe they can make even more money by putting it into something else (other riskier stocks, non stock investments like their own sole proprietor businesses, bitcoin, scratcher lottery tickets).
Some people are entirely risk averse. If they can't SEE their money they don't trust where it is so they buy precious metals or stack cash up. Neither of these are good investments for returns, but are generally safer that index investing (which is what S&P500 is) if you need to sell on short notice.
Investing anything requires money you don't have to spend somewhere else. Lots of people are at negative money, so they don't even have a dollar to invest.
A ton of people rely on the advice of financial advisors who don't have their interests in mind and who sell them mutual fund packages with high expense ratios that do poorly long term. These people generally lack the financial knowledge to know any different.
I was one of these. I started my IRA in my 20s with what little money I could put into it. When I left a job I'd roll my 401k back into my IRA under the same Edward Jones advisor.
After over more than 20 years I started questioning it. I asked for statements of all of my deposits. I took those dates and deposit amounts and plugged them into a basic historical simulator to see what would have happened if I put the same money into an S&P500 fund. My real investment account was over $40,000 lower than had I just put the money in myself into the S&P500. I dropped that advisor and transferred my entire balance into VTSAX and never looked back. Future deposits went into my own brokerage into boring index funds from then on.
I credit Edward Jones with making saving for retirement stupid easy for myself a dumb 22 year old at the time. However, I should have wised up sooner and it cost me at least $40,000 for my naïveté.
The stock market and investing (at least investing for the average person) is supposed to seem hard, and complex, and impossible to understand because how else are those thousands of mid-level Edward Jones employees and their ilk supposed to make a living?? Has no one thought of their incredibly easy white collar jobs??!!?
If you are currently in the process of saving instead of withdrawing in retirement, then falling stock prices are just buying opportunities. If the grocery store puts eggs on sale, you wouldn't fret that the eggs currently in your fridge aren't worth as much.
When you think of it that way, it gets a lot easier to hang on after a crash, and you might start looking for ways to buy even more at bargain prices.