In my experience:

• A lot of people do this with 401ks and such because many times there aren’t many other options.
• People I know who are serious investors with a lot of money tend to not invest much in the S&P 500 because they think of themselves as superior investors, but I don’t know of anyone for whom this is actually true based on past performance.
• I invest some 20% of my money in the S&P 500, which is probably not as much as I should. It’s some combination of the above hubris, which is natural, wanting to be diversified, and enjoying gambling on individual stocks.