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.
It's actually technically correct that we all can be millionaires, at least on a household basis. The mean household wealth in the US was $1.06 million as of 2022, by now it's undoubtedly higher. So with a full redistribution of wealth every household would have over $1 million.
In reality though the median household wealth is just under $200k as of 2022, and doesn't rise as consistently so who knows where it is now.