Everyone always quotes the growth of the S&P500, but isn’t pretty much no one 100% invested for their entire retirement in the S&P500?

Why does it matter if no one else does it? Investing is not a social experience. Most people don't do it because they are uninformed and ignorant about how to manage their money. The easy option is the easy option because you someone else can get more of a cut of your money. You generally pick up to two of these three with any product: good, easy, cheap. The promoted target date funds are usually just easy. They have high expense ratios and are therefore not good or cheap.